 |
Description  |
|
|
FIELD OF THE INVENTION
Polymerized surfactant-protein compositions and methods of their
preparation are provided for determining protein structure, providing for
biosensors, enzyme reactors and molecular electronics, and for providing
orderly arrays of proteins as base layers. Various parameters are employed
for achieving a variety of structural elements.
BRIEF DESCRIPTION OF THE BACKGROUND
In a universe where conditions direct substances to have a substantial
degree of disorder, there is a continuing interest in developing new
techniques for ordering molecules for a wide variety of purposes. In the
field of electronics, the ability to develop crystalline arrays having a
low level of imperfections has permitted ever increasing sophistication
and efficiency in the handling of electronic circuits. As efforts have
been made to increase the amount of information which may be stored or
manipulated within a particular volume or area, difficulties in defining
specific elements and preventing the interaction between adjacent elements
has greatly increased.
There is also substantial interest in being able to determine structures of
complex macromolecules. With proteins, it is frequently difficult to
obtain crystalline structures to permit structural determinations using
x-ray diffraction or two-dimensional crystals for use in electron
crystallography.
Other areas where order is of interest include structural members, such as
fibers, layers in multilamellar constructions, and the like.
There is also substantial interest in being able to produce novel
conducting materials, semiconductor materials and insulating materials,
particularly those that have properties which may be different from other
materials having analogous characteristics. The ability to employ switches
whose function can be modified by external conditions is of particular
interest and can have a wide variety of applications.
BRIEF DESCRIPTION OF THE RELEVANT LITERATURE
U.S. Pat. No. 4,489,133 describes procedures and compositions involving
orderly arrays of protein molecules bound to surfactants. Thomas et al.,
Electron. Letters (1984) 20:83-84 describe a GaAs/LB film MISS switching
device employing .omega.-tricosenoic acid as the surfactant bilayer for
producing a thin insulator. Lochner et al., Phys. Status Solidi (1978)
88:653-661 describe photoconduction in polydiacetylene multilayer
structures and single crystals. Sugi, J. Molecular Electronics (1985)
1:3-17 provides a review of Langmuir-Blodgett film use in electronics.
Reynolds, ibid (1986) 2:1-21 describes conducting organic polymers.
Wilson, Electron. Letters (1983) 19:237 describes the principles of a
three dimensional molecular electronic memory employing polydiacetylene
crystals or Langmuir-Blodgett multilayer films. Descriptions of electronic
devices employing organized macromolecular ensembles formed with
surfactant layer crystallization include Arrhenius et al., Proc. Natl.
Acad. Sci. USA (1986) 83:5355-5359; Haddon and Lamola, ibid (1985)
82:1874-1878; and Paleos, Chem. Soc. Rev. (1985) 14:45-67. Biosensors
employing surfactant layer crystallization are described by Owen, Ann.
Clin. Biochem. (1985) 22:555-564 and Thompson and Krull, Trends in Anal.
Chem. (1984) 3(7):173-178. Bader et al., Advances in Polymer Sci. (1985)
64:1-62 describe polymeric monolayers in liposomes as models for
biomembranes. See particularly, pages 22 to 25 describing polymerized
vesicles.
SUMMARY OF THE INVENTION
Novel articles are provided comprising at least two layers: (1) a
polymerized surfactant mono-layer having long polymeric strands in a
substantially ordered array; and (2) a protein layer bound to the
surfactant monolayer in a substantially ordered array. By varying
conditions in the preparation of the article, the conformation of the
article can be varied from substantially parallel multilayers to tubes,
filamentous arrays, helices or other complex assemblies. Parameters
affecting the molecules (surfactants-protein) which comprise the
assemblies may be altered in order to modulate the intrinsic physical
properties of the assembly. The articles can find use in protein structure
determination, electronics, biosensors, etc. Improved methods for
producing ordered protein arrays are also provided.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic view of a switching device;
FIG. 2 is an electron micrograph of cholera toxin crystals on layers of GM1
and polymerized tricosa-10,12-diynoic acid (1 cm is roughly 1000 .ANG.);
FIG. 3 is an electron micrograph of polymer filaments extending from
crystals of cholera toxin on GM1 and polymerized tricosa-10,12-diynoic
acid layers (1 cm is roughly 1000 .ANG.);
FIG. 4 is an electron micrograph of antibody bound surfactant-hapten layers
titrated with other filler surfactants (phospholipids) resulting in linear
tubes about 400-600 .ANG. in diameter. Negative stain outlines a central
channel which runs the length of each tube; and
FIG. 5 is an electron micrograph depicting intricately braided helical
wires tightly wound at the stem, branching along the length and splaying
out at the ends, resulting from the addition of multivalent cations to the
protein solution.
DESCRIPTION OF THE SPECIFIC EMBODIMENTS
Articles are provided and methods for their preparation, where the articles
are characterized by having at least two layers: (1) an ordered
polymerized surfactant monolayer; and (2) an ordered protein monolayer
bound to said surfactant monolayer by forces stronger than van der Waal
forces. By ordered is intended molecules in a regular lattice.
The articles may have two or more layers, there generally being from about
1 to 1000, more usually 1 to 100 surfactant layers, generally 1 to 10
layers, particularly 1 to 3 layers, while from 1 to 3, more usually 1 to 2
protein layers. The surfactant layers will generally be from about 10
.ANG. to 1.mu. in thickness or more, more usually from about 20 .ANG. to
0.5.mu. in thickness. The protein will generally be from about 25 to 1,000
.ANG. in thickness, usually from about 25 to 600 .ANG. in thickness.
Depending upon the nature of the article and the conformation of the
surfactant layer and the protein layer, thicknesses amy be varied widely.
The conformation of the article may be varied depending on the manner in
which it is produced. For some purposes, the layers will be stacked in
substantially parallel layers, which layers may be planar or have a
repeating cycle. Alternatively, the articles may have individual tubular
structures which may be independent or linked by covalent or non-covalent
means, where the tubes wil normally have an internal protein monolayer, a
surfactant layer, which may be mono- or multilamellar, and a protein layer
which may be mono- or multilamellar. A third configuration is a helical
configuration, where the surfactant layer and protein layer form an
entwined helix, with the appearance in an electron micrograph of being
wrapped around each other.
The surfactant layers may be crysalline, amorphous, or smectic crystals,
conducting or non-conducting, depending upon the nature of the surfactant
or surfactant mixture employed for preparing the surfactant layer. If
desired, the compositions of successive surfactant layers may be varied,
though for the most part the multilamellar surfactant layers will be of
substantially uniform composition.
The substantially parallel layered articles will usually have a minimum
planar dimension of 5 .ANG., particularly as a linear polymer. For the
most part, the surface areas of a layer will ususally be in range of about
10 .ANG..sup.2 to about 1 m.sup.2. The tubular structures will generally
range from about 25 to 1.mu. in diameter, will usually be at least about
500 .ANG. in length, more usually at least about 5.mu. in length. Helical
structures generally range from about 300 to 1,000 .ANG., in particular
cases structures having varying thicknesses of from about 200 to 2000
.ANG., in one example being observed at both about 400 .ANG. and at about
800 .ANG.. The length of the helix will generally be at least about
0.1.mu., usually at least about 1.mu., and may be 3.mu. or more, although
lengths greater than about 100.mu. are not observed.
New articles of this invention may be modified in a wide variety of ways.
Cross-linking may be provided, where the articles may be stacked one above
the other, or cross-linked in lateral dimensions, so as to provide arrays
of tubes or helical filaments. Crosslinking can be achieved in a variety
of ways, both covalent or non-covalent. Cross-linking agents include
glutaraldehyde, maleimidobenzoic acid, methyldithioacetic acid,
diazobenzoic acid bisulfite, antibodies, avidin, where biotin may be bound
to the protein or surfactant, etc. Thus, relatively large articles can be
produced which can be readily manipulated and used in a variety of ways.
The composition of the surfactant layer may be varied widely. The
composition may be homogeneous, where the bending surfactant is
polymerizable and has a polar terminus which may serve as a ligand for a
complementary binding protein, or heterogeneous where the binding
surfactant may or may not be polymerizable. The ligand surfactant may be
diluted with a polymerizable surfactant. Various functional groups may be
present to provide for polymerization. Illustrative groups include triple
bonds, monoynes, diynes, dienes, pyrroles, vinyl esters, vinyl ethers,
acrylates, vinyl aromatics, where the polymerizable functionality is part
of the surfactant chain or is attached to a surfactant chain. For the most
part, the surfactant chains will be aliphatic chains of at least 6,
usually at least 12 carbon atoms and not more than about 30 carbon atoms,
preferably being from about 14 to 26 carbon atoms, more preferably from
about 16 to 24 carbon atoms. While the unsaturated functionalities may be
linked to an activating functionality, such as carbonyl, oxy, aryl or the
like, usually the polymerizable functionalities will be separated from
both the polar and non-polar terminus, usually by at least 1 carbon atom
and generally from about 1 to 14 carbon atoms, more usually from about 1
to 12 carbon atoms.
Polymerizable surfactants have been extensively described in the literature
as is evidenced by the articles described above, whose disclosure relevant
to the preparation and use of polymerizable surfactants and the references
cited therein, are incorporated herein by reference.
A wide variety of surfactants may be employed, either by themselves or in
combinations, where the surfactant which provides for the specific binding
will be present in at least about 0.1%, usually about 5% and may be
present up to 100%, more usually from about 5% to 80%, desirably from
about 10% to 60%, more desirably from about 20% to 50%. The surfactants
may be salts, esters, either organic or inorganic, amides, ethers,
alcohols, cations, e.g. quaternary ammonium, or the like. The polar group
may be a carboxy salt, particularly multivalent salt, e.g. cadmium, lead,
barium, cobalt, manganese, etc., a sugar, an organic salt, e.g. betaine,
phosphate ester, e.g. phosphatidyl glycerol, serine, inositol, etc.
Naturally occurring, synthetic surfactants, or combinations thereof, which
may be employed in the formation of the surfactant layer include stearate,
oleate, linoleate, arachidonate, arachidate, vinyl stearate,
.omega.-tricosenoic acid, pentaeicosa-10, 12-diinoic acid,
.omega.-3'-pyrrolyloctadecanoate, cholesterol, gangliosides,
sphingomyelins, cerebrosides, etc.
For the most part, mixtures will be used where the specific binding
surfactant, which may or may not be capable of addition polymerization,
will be diluted with the surfactant capable of polymerization and in some
instances other surfactants as well to provide for specific properties.
For example, cholesterol may be added to introduce cholesteric properties
into the surfactant layer. For the most part, the mixtures will not have
more than about 6 different surfactants, usually not more than about 4
different surfactants and usually having from 2 to 3 different
surfactants. Of particular interest are those surfactants where the fatty
acids are joined to phosphatidychloine or phosphatidylethanolamine.
Alternatively, condensation polymerizable surfactants may be used, e.g.
amino acids, using water soluble carbodiimides as activators.
The protein layer bound to the surfactant layer may be any protein which
specifically binds to another molecule. Thus, the proteins may be
immunoglobulins, natural surface membrane receptors, such as growth factor
receptors, G-proteins, neurotransmitter receptors, et., blood protein
receptors such as thyroxine-binding globulin, enzymes, proteins which
recognize sugars, such as lectins, toxins, proteins which recognize
nucleic acids, such as histones, DNA and RNA polymerases, depressors, etc.
Thus, there is a rich availability of proteins which recognize specific
structures which can be joined to surfactants in a variety of ways to
provide for specific binding.
While the first layer requires a specific binding between the ligand bound
to the surfactant and the protein, various techniques can be used to add
additional protein layers, such as functionalizing the first protein layer
with, for example, biotin, and then providing a second layer of avidin.
Alternatively, antibodies can be employed which bind to the first lalyer
or other specific proteins which bind to the first protein layer, for
example, Staphyloccus aureus protein A or rheumatoid factor with
immunoglobulins. Thus, extended arrays can be obtained if desired, where
one wishes to have a plurality of protein layers.
The articles of this invention can be prepared, for the mostpart, using
conventional techniques employing particular conditions to achieve the
desired structures. For the most part, Langmuir-Blodgett techniques will
be employed as described in the references cited previously. In employing
the subject methods, attention should be given to the experimental section
for guidance as to the particular range that should be used with any
particular parameter for the desired result.
A large number of parameters are available which can be used to influence
the nature of the product. These parameters include the buffer, including
pH, ionic strength, cations employed, e.g. mono- or polyvalent,
composition of the surfactant, both as to the polymerizable surfactant and
the non-polymerizable surfactant, including such considerations as chain
length, the situs of the polymerizable functionality, the nature of the
polymerizable functionality, and the nature of the polar head group; the
manner in which the surfactant layer is formed, including concentration of
surfactant and solvent, the nature of the solvent, the spreading method,
and the amount of surfactant employed, which will affect the availability
for multi-lamellar layers; and physical parameters, such as film tension,
crystallization time, temperature, humidity, and E (electric) field, M
(magnetic) field (protein dipole moment). Also, there is the opportunity
to introduce various agents into the aqueous medium which can affect the
nature and conformation of the article, e.g. specific ions, organic or
inorganic.
An aqueous medium is formed, which is normally buffered at a pH in the
range of about 4 to 9, preferably from about 5 to 9. The salt
concentration will generally be in the range of about 10 mM to 1 molar.
Illustrative buffers include phosphate, borate, barbitone, carbonate,
Tris, MOPS, MES, etc. Illustrative buffer compositions include phosphate
buffered saline; 138 mM NaCl, 50 mM potassium phosphate, pH 7.2; 200 mM
sodium borate, pH 8.2. It is found that PBS favors monolayers, cadmium
stabilizes the layer, while borate favors multilayer packing. In addition,
various organic cations may be added such as polyamines, where the amino
groups tend to be separated by at least 2 carbon atoms and not more than
about 12 carbon atoms. Illustrative compounds include sperimine,
spermidine, nopaline, octopine, etc. The presence of the polyvalent
organic cations, as well as relatively high levels of inorganic cations,
e.g. 50 mM magnesium, or higher, will favor helical structures and
paracrystalline arrays. The concentration of the multivalent cations will
depend to some degree upon the nature of the cation, generally ranging
from about 0.1 to 200 mM, more usually from about 10 to 100 mM and will be
included in the determination of total salt concentration.
The protein may be introduced into the aqueous medium either before or
after formation of the surfactant layer. When it is introduced after
formation of the surfactant layer, it will be injected carefully below the
surfactant layer from the side, so as not to distrub the orientation of
the surfactant layer. The concentration of the protein will vary widely,
usually being at least stoichiometric with the amount of available
complementary ligand. That is, where the surfactant layer is homogeneous
having only ligand bound surfactant, only a proportion of the total ligand
will be available for binding. Depending on the nature of the protein,
this will generally vary from about 0.1 to 0.5 of the total ligand
available. If desired, substantial stoichiometric excess may be employed,
usually about 1.5 stoichiometric, more usually about 2. The excess may be
five-fold or higher. The amount employed will also be affected by the
binding affinity of the protein for the ligand, the size of the protein,
the effect of the number of protein molecules on the nature of the order
of the array, and the like. Generally, concentration of protein will be
from about 10 .mu.g/ml, not more than 5 mg/ml, more usually not more than
about 1 mg/ml, more usually in the range of about 500 .mu.g/ml.
In preparing the surfactant layer, surfactant will be dissolved in a
convenient non-polar volatile solvent. Illustrative solvents include
chloroform, hexane, isopropyl ether, methylenedichloride, benzene, etc.
Individual solvents or combinations may be employed, depending upon the
nature of the surfactant. Trace amounts of an organic hydrophillic solvent
may be employed, when necessary to solublize a particular surfactant. The
concentration of surfactant will generally be from about 0.01 to 5, more
usually about 0.1 to 1 mg/ml.
The aqueous medium may be in any convenient container, particularly a
plastic container, such as Teflon, PVC, Delrin, glass or metal. Various
containers are available for the formation of surfactant monolayers which
allow for the control and monitoring of film pressure. The surfactant will
usually be maintained at at least the maximum equilibrium spreading
pressure and up to the collapse pressure.
It may be desirable to add the protein slowly to the surfactant layer. In
this way, the size of the ordered or crystalline domains may be enhanced.
One way to achieve this result would be the slow control of addition of
protein, using various types of pumps or motorized controlled syringes for
introducing the protein into the buffered aqueous medium. Alternatively,
one may use two wells joined by a bridge, e.g. a bibulous layer, which
allows for transfer of surfactant from one well to the other. By spreading
surfactant in one well and having the aqueous protein containing buffered
medium in the other well, the surfactant may be transferred at a
controlled rate from the first well to the second well by forming ordered
surfactant layers and then continuously increasing the pressure forcing
the surfactant to migrate across the bridge to the other well. By
monitoring and controlling the pressure in the second well, an ordered
layer can be maintained, which will react with protein in the aqueous
buffered medium as it spreads on the surface in the second well.
The surfactant layer will initially be held at at least the maximum
equilibrium spreading pressure, but may be changed, higher or lower during
protein binding, while still maintaining a condensed surfactant phase.
The system will now be incubated for at least about 5 minutes to insure
reaction between the protein and the surfactant. Usually the reaction will
not require more than 48 hours, more usually not nore than about 24 hours.
In this period, the protein will bind and become ordered, where the
protein may assume its lowest thermodynamic state as bound to the
surfactant. A humid atmosphere is normally maintained during the reaction.
Depending upon the nature of the polymerizable functionality, the
surfactant may now be polymerized. The polymer may have as few as 3 units
or may have 10.sup.7 or more units. Alternatively, the surfactant could be
polymerized prior to contact with the protein if desired. In either event,
the same techniques for polymerization would be used. Preferably,
polymerization is carried out after conjugation with the protein.
Polymerization may be achieved by employing short wave ultra violet light,
e.g. below 300 nm, with diynes, in the range of about 230 to 275 nm,
X-rays, free radicals, redox agent, or other convenient initiator. For the
most part, the polymers will be addition polymers rather than condensation
polymers, although condensation polymerization may be employed as
indicated previously. The time for the polymerization, for example
irradiation, will be at least about 1 min., and usually not more than
about 60 min., frequently not more than about 30 min. Upon completion of
the polymerization, the article is now formed and may be used in a variety
of ways, depending upon its conformation.
Where a multilamellar surfactant layer is desired, this can be achieved by
employing an excess of surfactant over that required for monolayer. Thus,
by raising the pressure to just below the maximum equilibrium spreading
pressure, generally less than about 10% below the maximum equilibrium
spreading pressure, mono- or multilamellar films can be obtained.
Alternatively, successive dippings using Langmuir-Blodgett methods may be
employed for multilayers.
It is found that the lateral surface tension of the surfactant layers
exerts similar effects on crystallization of antibodies, ribonucleotide
reductase and cholera toxin, indicating relatively low dependence on
protein type. By contrast, the concentration of surfactant-ligand in the
surfactant layers and the type of other surfactant lacking ligand (filler
surfactants) had varying effects on array formation among the different
proteins.
The temperature for the reaction may vary widely, generally being below
about 60.degree. C., preferably being below about 40.degree. C., usually
ranging from about 0.degree. C. to 30.degree. C., more usually from about
40.degree. C. to 30.degree. C.
The subject method can be used without polymerizable surfactants and
polymerization to obtain improved ordered protein arrays.
Once the structure has been formed, as already indicated it may be modified
by cross-linking, covalently or non-covalently, using various
polyfunctional agents, e.g. immunoglobulins, cross-linking agents, high
energy irradiation, or the like. The article may be retrieved in a variety
of ways. Vertical dipping may be employed, where a solid substrate may be
introduced into the aqueous medium while the film is maintained under
pressure, so as to coat the solid substrate as it enters and is retrieved
from the aqueous medium.
Alternatively, horizontal lifting methods can be employed, where the
substrate is held horizontally and is lowered onto the monolayer on the
water surface from above. Alternatively, the substrate may be immersed in
the water before the spreading of the monolayer and contact is made with
the protein layer, as the substrate is raised through the aqueous medium
comes in contact with the protein layer. The techniques include the
monomolecular, the sweeping technique, use of polymers such as polyvinyl
alcohol to provide a polymeric coating, which may then be removed with
water, or the like.
The subject articles are found to have a high degree of stability for use
in electron microscopy. Thus, the subject articles are substantial
improvements over other techniques which have been previously described to
obtain electron micrographs of organized linear protein arrays. For this
purpose, carbon-coated electron microscope (EM) grids are contacted with
the hydrophobic tails of the sufactant adjacent the air/water interface,
the grids removed and stained for electron microscope investigation.
The subject articles, being surfactant polymer layers decorated with
enzymes, may be used in the construction of highly efficient enzyme
reactors. The polymer surface, when saturated with specifically bound
enzyme, will yield the maximum concentration of catalyst in two
dimensions. Because of the high degree of structural integrity these
assemblies may be placed in rapid flow devices in a variety of
orientations without suffering structural damage.
The subject articles may also be used in a variety of electronic sensors
and devices. A particular application is described by Thomas et al. (1984)
supra, where the subject article comprising the composite
surfactant-protein layers may be substituted for the thin insulator
described by Thomas. Similarly, the subjet layers may be used as
substitutes for the polydiacetylenes described by Wilson, (1983) supra. In
the Wilson article, Wilson describes the ability to read energy into the
system and read out fluorescence. By having proteins present, by varying
the medium in which the protein is in contact, changes in the conformation
of the protein will result in changes in the electronic nature of the
polymeric surfactant layer. Thus, variations in wave length of the emitted
light can be related to specific materials present in the medium.
The variations in the conformation of the proteins could be as a result of
a variety of effects. For example, with enzymes, binding of substrates or
inhibitors could result in a change in conformation. For a large variety
of proteins, allosteric effects are involved, particularly proteins which
regulate various nucleic acid processes, such as replication and
transcription. Also, enzymes are known to be subject to allosteric
effects. In addition, various membrane proteins or membrane binding
proteins, particularly receptors, will be subject to modification upon
binding to ligand. The possibilities include membrane associated proteins,
e.g. cholera toxin, water soluble proteins and integrated membrane
proteins, e.g. ion channel proteins, and nucleic acid binding to proteins,
such as histones, polymerases, repressors, or the like.
A device which may be considered is depicted in FIG. 1. Device (10) is a
semiconductor layer (12), which is etched to define wells (14) and islands
(16), to provide insulation for individual islands. By appropriate
polarization, each of the islands may be insulated from the other islands.
Mounted on the surface of each of the islands (16), is a surfactant layer
with a dilamellar layer being depicted (18) having a first layer (20) and
a second layer (22), where the second layer (22) contains the ligands
(ligands may be in one or more layers). Protein layer (24) is bound to the
ligands of second layer (22) to define in this example, a protein
insulating layer which separates metal layer (26) from the polymerized
surfactant layer (22). Layer (20) may be the same as layer (22) and may
provide a polymerized layer or non-polymerized layer, usually being a
polymerized layer. The thin metal may be evaporated onto the protein layer
to provide for a conductor.
In practicing the invention, a potential is provided across semiconductor
(12) and metal layer (26). Light pulses may then be introduced which are
transmitted through metal layer (26) and penetrate to at least surfactant
layer (22). Depending upon the composition of the surfactant polymer
layer, the fluorescent light emitted by the electronic array will vary.
Thus, by subjecting the proteins to various molecules, such as effector
ions or the like, or varying physical paramaters which may affect their
conformation, one can modulate a change in wavelength of the observed
fluorescent light. Furthermore, by isolating the individual island arrays,
one from the other, each of the arrays may be subjected to different media
and individually interrogated by irradiating an individual array with
light, while maintaining the other arrays in the dark.
The subject articles can be used a photoconductors, as described by Lochner
et al., Phys. Stat. Sol. (1978) 88:653-661. The presence of the proteins
stabilize the structure of the conductive polymers and provide insulation
of the polymeric chains, and can be used to modulate the photoconduction
by virtue of their response to temperature, various media, or agents in
the media which may bind, specifically or non-specifically, to the
proteins.
Dealing with tubular arrays or helices, where the tubular arrays or helices
aggregate, various techniques may be employed for separating individual
members, so as to provide for interactions with the individual members.
For example, the arrays may be placed on an ice surface, where the ice
forms a convex layer. It is found that the surface tension will provide
for separation of the individual members. The separated ends may then be
bound to a conductive or semiconductive surface and may be individually
interrogated by light pulses. In this manner, the presence or absence of
light at a particular site may be determined, which can be used for the
manipulation of information.
The following examples are offered by way of illustration and not by way of
limitation.
EXPERIMENTAL
Studies with the E. coli B1 dimer of Ribonucleoside Diphosphate Reductase
Surfactants
For synthesis of dATP-aminocaproyl-PE
(PE-dioleoylphosphatidylethanolamine), the triethyl ammonium salt of
2'-deoxy-5'-adenosine triphosphate was activated with
dicyclohexylcarbodiimide and allowed to combine with aminocaproyl-PE.
Other surfactants were purchased from Avanti Polar Lipids.
Surface Pressure Measurements
Teflon or glass troughs (1 mm depth, 3 cm.sup.2 surface area) were filled
with water or buffers and varying amounts of dATP-aminocaproyl-PE and egg
PC (phosphatidylcholine) (0.5 mg/ml, chloroform:hexane 1:1 (v/v)) were
applied to the surface. Lateral pressures were monitored with time by the
Wilhelmy plate method (Mobius et al., Ber. Bunsenges. Phys. Chemil. (1969)
73:845.
Crystallization Trials
Drops of B1 (E. coli B1 subunit of ribonucleoside diphosphate reductase)
solution (1-3 .mu.g of protein in 12 .mu.l of buffer) in teflon wells (4
mm in diameter, 1 mm deep) were coated with surfactant mixture (0.25-0.5
.mu.l, 0.5 mg/ml in chloroform:hexane, 1:1 (v/v)) and kept in a humid
atmosphere at 4.degree.-30.degree. C. Buffers contained 100 mM NaCl, 1-100
mM MgCl.sub.2, 2 mM dithiothreitol, and either 25 mM MES-Na.sup.+, pH
5.4-6.9, or 25 mM Tris-HCl, pH 7.1-8.8. After 1-14 days, carbon-coated
electron microscope grids were touched to the surface, washed with one
drop of water or 10 mM Tris-HCl, pH 7.4, and stained with 1% uranyl
acetate.
The formation of surfactant layers containing dATP-aminocaproyl-PE was
monitored by measurements of surface pressure at the air-water interface.
Mono-layers of pure dATP-aminocaproyl-PE spread on water showed a similar
pressure area profile to dioleoylphosphatidylcholine (dioleoyl PC), with
collapse upon compression to about 48 dynes/cm. Salts in the subphase
(0.1-0.3M NaCl, neutral pH, with or without 5 mM MgCl.sub.2) caused
instability of dATP-aminocaproyl-PE layers spread on a fixed surface area.
Collapse occurred within minutes of formation at all pressures tested
(13-48 dynes/cm). The difficulty was overcome by adding other surfactants.
A mixture of egg PC and dATP-aminocaproyl-PE in a 9:1 molar ratio formed
monolayers as stable as those of pure egg PC, and films of this
composition were used to crystallize the B1 dimer.
Crystals were formed at the air-water interface. Drops of B1 solution (2-3
.mu.g of B1 protein in 12 .mu.l solution containing 100 mM NaCl, 15 mM
MgCl.sub.2, 15 mM spermidine, 1.5 mM dTTP) in teflon wells were coated
with surfactant mixture, kept in a humid atmosphere at 23.degree. for 1-46
hrs, and examined by touching the surface with a carbon-coated grid,
washing, negative staining, and electron microscopy (Philips 201
transmission electron microscope). Within hours, linear arrays appeared
which showed some tendency to pack side by side but little long range
order. After 20 hrs, two-dimensional crystals were observed. Omission of
dATP-aminocaproylPE or the addition of 1.5 mM dATP gave no crystals, and
no evidence of protein binding (a studded appearance of the surfactant
layers). Omission of dTTP gave fewer ordered arrays even upon longer
incubation. Arrays formed at pH values between 5.7 and 8.0, with an
optimum in the degree of order near pH 7.4. Under more acidic conditions,
the surfactant layers appeared distorted, while under more alkaline
conditions, there was extensive protein binding but no ordered arrays. The
composition of the surfactant layers was crucial. Substitution of egg PE
for PC gave protein binding but no ordered arrays. Egg monomethyl and
dimethyl PE gave arrays, but the degree of order appeared less than with
PC.
Crystals gave optical diffraction patterns that could be indexed on a
rectangular reciprocal lattice, with unit cell parameters of a=277 .ANG.,
b=110 .ANG., and .gamma.=90.degree.. The diffraction extended to a 37
.ANG. resolution. Filamentous arrays and helical bundles of filaments were
favored with increasing amounts of magnesium (.ltoreq.100 mM) in the
buffers.
Image analysis (Amos et al., Prog. Biophys. Molec. Biol. (1969) 38:183-231)
of rectangular crystals was carried out with 38 independent reflections.
Fourier synthesis gave "noise-filtered" images showing features suggestive
of two-fold rotational symmetry. On refining the origin of the reciprocal
lattice by minimizing the differences in phase between two-fold related
reflections, an average difference from centrosymmetric phases of
12.degree. was obtained for seven averaged data sets. Fourier synthesis
gave a symmetry-averaged image showoing four elongated protein densities
in the unit cell, each about 110 .ANG. long and 32 .ANG. in width. Two
objects paired together, suggesting, based on these dimensions, that a
pair of objects is a B1 dimer.
STUDIES WITH CHOLERA TOXIN B SUBUNIT
Materials and Methods
Materials
Cholera toxin B subunit, purified from culture filtrates of V. cholerae
strain 569B by affinity chromatography (Toyot et al., Eur. J. Biochem.
(1981) 113: 249-258), was provided by the Institute Merieux, Marcy,
France. B subunit was also purchased from Sigma Chemical Co. Egg
phosphatidylcholine and dioleoylphosphatidylethanolamine were from Avanti
Polar Lipids, Inc., and ganglioside GM1 was from Supelco.
Bis(sulfosuccinimidyl)suberate and dimethylsuberimidate were from Pierce
Chemical Co. and Sigma Chemical Co.
Crystallization, Electron Microscopy and Image Analysis
Surfactant layers were spread at the airwater interface by applying 1.2
.mu.l of surfactant solution (0.5 mg/ml in chloroform:hexane, 1:1 by
volume) to the surface of a 50 .mu.l drop of aqueous solution (10 mM Tris,
150 mM NaCl, pH 7.3) containing B subunit (250 .mu.g/ml solution) in a
teflon well. After 4 to 48 h at room temperature, a carbon coated electron
microscope grid was brought in contact with the drop, and the grid was
washed with buffer and stained with 1% uranyl acetate. Micrographs were
recorded with minimal electron doses on a Philips EM 400 microscope
operating at 100 kV. Optical diffraction was used to assess the quality of
the images. Image processing was performed according to Amos et al., Prog.
Biophys. Molec. Biol. (1982) 39: 183-231.
Chemical Cross-Linking
B subunit (40 .mu.l of a 1.0 mg/ml solution in 0.2 M sodium borate, pH
8.5), was treated with various concentrations of
bis(sulfosuccinimidyl)suberate at room temperature. After 45 min, the
mixture was boiled for 10 min and cross-linked products were resolved by
electrophoresis in an SDS-15% polyacrylamide gel (Weber and Osborn, J.
Biol. Chem. (1969) 244:4406-4412). Alternatively, 0.4 ml of B subunit (1
mg/ml) was mixed with 0.5 ml of liposomes (egg phosphatidylcholine: GM1,
in a 9:1 molar ratio, sonicated for 5 min in a bath-type sonicator at 2 mg
of total surfactant per ml of 0.2 M sodium borate, pH 8.5) and, after 30
min at room temperature, the mixture was cross-linked.
RESULTS
Two-Dimensional Crystals of B Subunit-Ganglioside Complexes
Surfactant layers were prepared from dioleoylphosphatidylethanolamine or
egg phosphatidylcholine, mixed with 10 mole % GM1, by coating the mixture
on drops of aqueous solution. Five to ten-fold more surfactant was used
than necessary to form a monolayer, to ensure the maintenance of maximum
equilibrium spreading pressure of the surfactant film, a condition for
optimal yields of ordred protein arrays. B subunit was present in the
aqueous solution before application of the surfactant mixture or,
alternatively, it was injected beneath the surface of preformed surfactant
layers to avoid denaturation of protein at the airwater interface. Protein
binding occurred rapidly (shown by a studded appearance of surfactant
layers in the electron microscope), followed by crystallization within 6 h
at room temperature. The largest crystalline areas were obtained with
phosphatidylethanolaminecontaining films after about 24 h of incubation.
Typically, 10-20% of the surface of an electron microscope grid was
covered with crystals up to 15.mu. on a side.
Projected Structure of the B Oligomer
Two crystal forms were revealed by optical diffraction from electron
micrographs, a rectangular lattice (unit cell dimensions a=120 .ANG.,
b=131 .ANG., .gamma.=90.degree.), which gave diffraction extending to a
ninth order reflection at 15 .ANG. resolution and a hexagonal lattice
(unit cell dimension 68 .ANG.) which gave four orders of diffraction,
corresponding to 19 .ANG. resolution. The proportion of the two crystal
forms varied, with no discernible dependence of pH, ionic strength, time
or temperature.
Projected structures were computed with the use of 38 and 15 independent
reflections from the rectangular and hexagonal lattices, corresponding to
17 and 28 .ANG. resolution. (Origin refinement gave average and root mean
square phase residuals of 12.5.degree. and 8.0.degree. in the two cases.)
The structures showed rings of five or six protein densities, nearly
identical in outer diameter (60 .ANG.), inner diameter (20 .ANG.), and
distance from the center of the right to a peak of protein density (20
.ANG.).
Chemical Cross-Linking of the B Oligomer
The occurrence of two crystal forms, apparently containing different
oligomeric structures of the B subunit, was reinvestigated by chemical
cross-linking. In previous studies with dimethyl suberimidate as
cross-linking reagent, five bands were resolved in SD-polyacrylamide gels,
indicative of a pentameric structure (Cull, Biochem. (1976) 15:1242-1248).
With bis(sulfosuccinimidyl)suberate, a pattern of six bands was obtained.
whose mobilities in SDS-polyacrylamide gels fall on a smooth curve in a
semilog plot against the molecular weights of B polypeptide monomer
through he | | |
