Freeze-fractured purple membrane particles: protein content

Supramolecular organization of rhodopsin in rod photoreceptor cell membranes

Rhodopsin is the light receptor in rod photoreceptor cells that initiates scotopic vision. Studies on the light receptor span well over a century, yet questions about the organization of rhodopsin within the photoreceptor cell membrane still persist and a consensus view on the topic is still elusive. Rhodopsin has been intensely studied for quite some time, and there is a wealth of information to draw from to formulate an organizational picture of the receptor in native membranes. Early experimental evidence in apparent support for a monomeric arrangement of rhodopsin in rod photoreceptor cell membranes is contrasted and reconciled with more recent visual evidence in support of a supramolecular organization of rhodopsin. What is known so far about the determinants of forming a supramolecular structure and possible functional roles for such an organization are also discussed. Many details are still missing on the structural and functional properties of the supramolecular organization of rhodopsin in rod photoreceptor cell membranes. The emerging picture presented here can serve as a springboard towards a more in-depth understanding of the topic.

The Freeze-Fracture Technique: Application to the Study of Animal Plasma Membranes

During recent years, the freeze-fracture (FF) technique has become one of the most useful procedures available for the ultrastructural analysis of cell components, particularly for the study of biological membranes. The method has gradually evolved from a highly specialized and technically complex procedure to a reasonably accesible one, mainly as a consequence of improvements in commercial FF equipment, the understanding of the fracturing process and the artifacts induced, and the development of ancillary techniques for the study of cell membrane organization. Due to these advances, the FF method can be considered at present as an almost standard procedure for biological electron microscopical laboratories.

UHV freeze-facturing and image processing applied to the purple membrane

The complementarity of periodic features on the fracture faces of the yeast plasmalemma has been established to a resolution 20Å by ultrahigh vaccum (UHV, p≤10-9Torr) freeze-fracturing at -196°C and subsequent digital image processing. The structural record is considered reliable because of the one-to-one correspondence of volcano-like practicles on the plasmic fracture face (PF) and of ring-like depressions on the extraplasmic face (EF). The PF-particles were tentatively related to intrinsic proteins. Nevertheless experience with specimens which are biochemically and structurally more well defined than yeast appears indispensable in learing how to interpret the structural record. For this purpose the purple membrane of Halobacterium halobium appeared attractive.

Change of purple membranes geometry induced by protein adsorption

Earlier it was an orthodoxy that purple membranes (PMs) in aqueous medium are shaped as flat hard disks. In a few newer articles it has been shown that PMs are bent and their curvature varies with surface charge density. The purpose of this work is to answer which is the dominant factor for PM bending--structural or electrostatic forces. Two positively charged proteins are used: phytohemagglutinin (PhHA) and protamine. The electrophoretic mobility and electric polarizability of PMs are measured by microelectrophoresis and electric dichroism. The results show that both proteins reduce the mobility because they are adsorbed on PM surface. However, their influence on the electric polarizability is in the opposite direction--protamine reduces it (trivial effect) while PhHA increases the polarizability (non-trivial effect). The last result is explained by a straightening the initially bent PM because of specific bonding of PhHA to asymmetrically disposed glycolipids of PM in contrast to the electrostatic adsorption of protamine. It has been concluded that PMs in water medium are bent in the same manner as in in vivo--the intracellular surface with a higher negative charge is concave. The results indicate that electrostatic forces play a significant role in PM curvature but the shape of structural elements is the main factor determining the geometry of PM.

Monitoring expression and clustering of the ionotropic 5HT3 receptor in plasma membranes of live biological cells

The ionotropic 5HT(3) receptor was expressed in transiently transfected mammalian cells, yielding an unprecedented high concentration of up to 12 million receptors per cell. Receptor traffic in the plasma membrane of live cells was observed continuously over 24 h by fluorescence scanning confocal microscopy. This was possible by using 5HT(3) receptor-specific fluorescent ligands with high binding affinity and low off-rate to pulse label receptors at any time after appearance on the cell surface, and label subsequently those receptors expressed later by another, spectrally distinguishable, high-affinity fluorescent ligand. Having reached a critical cell surface concentration of approximately 3000 receptors/microm(2), the receptors started to aggregate in patches with a 4-fold increased surface concentration. The clusters were constantly delivered from a pool of freshly expressed receptors isotropically distributed within the basolateral region of the cell membrane. From there, they migrated to and accumulated on the apical cell surface approximately 9 h after transfection. Individual clusters grew until they reached a critical size of 1-2 microm when they merged to form with 3-5 microm large macroclusters. Clustered receptors were immobile on the minute time scale but always coexisted with monomeric receptors in the regions surrounding the clusters as revealed by fluorescence correlation spectroscopy. Because the receptor density of 12 000 receptors/microm(2) in the patches is as high as that found in two-dimensional crystals of certain membrane proteins, such patches might be a proper source for direct crystallization of membrane proteins without prior purification.

Functional expression of green fluorescent protein derivatives in Halobacterium salinarum

We investigated the applicability of the green fluorescent protein (GFP) of Aequorea victoria as a reporter for gene expression in an extremely halophilic organism: Halobacterium salinarum. Two recombinant GFPs were fused with bacteriorhodopsin, a typical membrane protein of H. salinarum. These fusion proteins preserved the intrinsic functions of each component, bacteriorhodopsin and GFP, were expressed in H. salinarum under conditions with an extremely high salt concentration, and were proved to be properly localized in its plasma membrane. These results suggest that GFP could be used as a versatile reporter of gene expression in H. salinarum for investigations of various halophilic membrane proteins, such as sensory rhodopsin or phoborhodopsin.

Protein structural change at the cytoplasmic surface as the cause of cooperativity in the bacteriorhodopsin photocycle

The effects of excitation light intensity on the kinetics of the bacteriorhodopsin photocycle were investigated. The earlier reported intensity-dependent changes at 410 and 570 nm are explained by parallel increases in two of the rate constants, for proton transfers to D96 from the Schiff base and from the cytoplasmic surface, without changes in the others, as the photoexcited fraction is increased. Thus, it appears that the pKa of D96 is raised by a cooperative effect within the purple membrane. This interpretation of the wild-type kinetics was confirmed by results with several mutant proteins, where the rates are well separated in time and a model-dependent analysis is unnecessary. Based on earlier results that demonstrated a structural change of the protein after deprotonation of the Schiff base that increases the area of the cytoplasmic surface, and the effects of high hydrostatic pressure and lowered water activity on the photocycle steps in question, we suggest that the pKa of D96 is raised by a lateral pressure that develops when other bacteriorhodopsin molecules are photoexcited within the two-dimensional lattice of the purple membrane. Expulsion of no more than a few water molecules bound near D96 by this pressure would account for the calculated increase of 0.6 units in the pKa.

The essential role of specific Halobacterium halobium polar lipids in 2D-array formation of bacteriorhodopsin

The mechanism whereby bacteriorhodopsin (BR), the light driven proton pump from the purple membrane of Halobacterium halobium, arranges in a 2D-hexagonal array, has been studied in bilayers containing the protein, 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC) and various fractions of H. halobium membrane lipids, by freeze fracture electron microscopy and examination of optical diffractograms of the micrographs obtained. Electron micrographs of BR/DMPC complexes containing the entire polar lipid component of H. halobium cell membranes or the total lipid component of the purple membrane, with a protein-to-total lipid molar ratio of less than 1:50 and to which 4 M NaCl had been added, revealed that trimers of BR formed into an hexagonal 2D-array similar to that found in the native purple membrane, suggesting that one or more types of the purple membrane polar lipids are required for array formation. To support this suggestion, bacteriorhodopsin was purified free of endogenous purple membrane lipids and reconstituted into lipid bilayer complexes by detergent dialysis. The lipids used to form these complexes are 1,2-dimyristoyl-sn-glycerol-phosphocholine (DMPC) as the major lipid and, separately, each of the individual lipid types from the H. halobium cell membranes, namely 2,3-di-O-phytanyl-sn-glycero-1-phosphoryl-3'-sn-glycerol 1'-phosphate (DPhPGP), 2,3-di-O-phytanyl-sn-glycero-1-phosphoryl-3'-sn-glycerol 1'-sulphate (DPhPGS), 2,3-di-O-phytanyl-sn-glycero-1-phosphoryl-3'-sn-glycerol (DPhPG) and 2,3-di-O-phytanyl-1-O-[beta-D-Galp-3-sulphate-(1----6)-alpha-D- Manp-(1----2)-alpha-D-Glcp]-sn-glycerol (DPhGLS). When examined by freeze-fracture electron microscopy, only the complexes containing 2,3-di-O-phytanyl-sn-glycero-1-phosphoryl-3'-sn-glycerol- 1'-phosphate or 2,3-di-O-phytanyl-sn-glycero-1-phosphoryl-3'-sn-glycerol-1'-sulphate, at high protein density (less than 1:50, bacteriorhodopsin/phospholipid, molar ratio) and to which 4 M NaCl had been added, showed well defined 2D hexagonal arrays of bacteriorhodopsin trimers similar to those observed in the purple membrane of H. halobium.

Effects of retinyl acetate on surface morphology and intramembrane particle distribution in the plasma membrane of 10T1/2 cells

Scanning electron microscopy and freeze fracture electron microscopy were used to characterize membrane ultrastructural differences between parental, C3H/10T1/2, and carcinogen-initiated, INIT C3H/10T1/2, cells and treatments with retinyl acetate. The intramembranous particle distribution on the E-face was detected and quantitated by the methods of automated image analysis to obtain statistically meaningful numerical characteristics of intramembranous particle size and density. Subtle differences were found when no differences were apparent by light microscopy or by scanning electron microscopy. Initial retinyl acetate treatment caused a significant increase of the intramembranous particle size in parental cells. Intramembranous particle density increased for retinyl acetate treatment in parental and INIT cells and in INIT cells previously maintained but withheld from retinyl acetate. Intramembranous particle distribution analysis includes the interparticle distance of nearest neighbors and the randomness of the distribution by the differential density distribution function, which compares the observed sample to Poisson modified for particle size. These measures show that the three cell groups that have been treated with retinyl acetate have a more even distribution of intramembranous particles than was found for untreated parental cells. The relationship between the freeze fracture morphology and the biological responses to retinyl acetate treatment is discussed.

Ultrastructure of the apical zone of Euglena gracilis: photoreceptors and motor apparatus

Euglena is an organism that every student of biology has observed; its morphology has been a subject of interest since the early microscopic literature for its enigmatic role of "plant-like" or "animal-like" organism. Therefore, this review has no pretensions to absolute novelty, but, like a journey to the centre of the earth, will attempt to arouse the reader's curiosity by taking him inside the cell Euglena, through the canal opening into the reservoir chamber. In light of the most recent knowledge, though much remains to be clarified, the aim is to provide information from ultramicroscopical studies on the apical zone of Euglena and possible functional meanings of the structures present therein. The survey of these structures is carried on as a study in correlation: TEM of cells after various treatments is correlated with SEM of cells fixed by means of different techniques. Notes on locomotion and other features of cytological and biological interest are added to assist with the comprehension of this microorganism.

Scanning tunneling microscopy of planar biomembranes

We combined planar membrane monolayer techniques with scanning tunneling microscopy (STM) to measure the thickness of metal-coated purple membrane (PM) isolated from Halobacterium halobium. Although the metal coating precluded obtaining high-resolution lateral information, it facilitated obtaining high-resolution vertical information. For example, the apparent mean thickness of planar PM and variations in thickness of enzyme-treated PM could be detected and quantified at sub-nanometer resolution.

Monolayer freeze-fracture and scanning tunneling microscopy

This article reviews research on planar monolayer methods, application of the methods to analyses of transmembrane signaling, and the combination of these methods with scanning tunneling microscopy (STM). Past research has involved the development of monolayer freeze-fracture methods. These include monolayer freeze-fracture autoradiography (MONOFARG), an electron microscopic cytochemical method to analyze in-plane distributions of radioisotopes, and double-labeled membrane splitting (DBLAMS) and single-membrane monolayer splitting (SMMS), methods to analyze transmembrane distributions of native and radiolabeled proteins and lipids. Present research has focussed on using these methods to investigate mechanisms of transmembrane signaling mediated by protein kinase C (PKC), including the transbilayer distribution of the tumor promoter TPA, a lipophilic activator of PKC, and the transbilayer distribution of peripheral membrane proteins phosphorylated by PKC. Future work will involve the combination of planar sample preparation with STM. The principles and applications of biological STM are briefly reviewed and a low-resolution STM image of a planar purple-membrane monolayer is included. The combination of planar methods and STM can provide the chemical information lacking in STM images enabling microscopists to investigate biochemical phenomena at nanometer resolution.

Photoreceptor disk membranes of Lampetra japonica

With the aim of characterizing photoreceptor outer segments and obtaining in situ observation of macromolecular variations due to cell types as well as adaption, we counted the number of outer segment disk membranes using electron micrographs of ultrathin sections as well as intramembrane particles on the complementary replicas of the retina of Lampetra japonica. Long photoreceptor cells (LPCs, cone-type cells) numbered 10,000/mm2 in the central as well as peripheral regions, while short ones (SPCs, rod-type cells) numbered 30,000/mm2 in the same regions. The LPC outer segment exhibited 306 disks on average during the light cycle versus 364 during the dark cycle. 12.0% of the LPC disks during the light cycle versus 13.4% during the dark cycle represented the "open" disks. The SPC outer segment exhibited 470 disks on average during the light cycle versus 507 during the dark cycle. 11.1% of the SPC disks during the light cycle versus 13.6% during the dark cycle represented the "open" disks. The LPC disk membrane contained 44.3 particles/0.01 microns 2 during the light cycle versus 39.5 particles during the dark cycle, 95% of which were derived from the protoplasmic fracture (PF) face. The SPCs contained 36.0 particles/0.01 micron 2 during the light cycle versus 43.6 during the dark cycle, 90% of which were derived from the PF-face. The present findings contradict the frequently cited hypothesis that an "open" disk, retaining continuity with the plasmalemma, is preserved characteristically into later stages by the cone outer segment. The significance of the intramembrane particles for the activity of the photoreceptor membrane is discussed.

Immuno-labelling for freeze-fracture: application to a cell surface attachment antigen

The cell-surface attachment (CSAT) antigen was investigated by using a technique that combines freeze-fracture with thin-sectioning. Chick heart cells were labelled using colloidal gold secondary antibodies, then fractured and shadowed with platinum vapor. The cells were dehydrated and embedded with their gold labels and replicated surfaces still intact. Sectioning revealed gold particles superimposed with external (E) fracture faces. The gold was definitely not coincident with E face particles and did not appear to be associated with pits. Therefore, it can be concluded that the CSAT glycoproteins is not represented by an E face particle. Whether it may be seen as an E face pit, or is not visible at all in freeze-fracture, is still an open question.

Formation of the apical pole of epithelial (Madin-Darby canine kidney) cells: polarity of an apical protein is independent of tight junctions while segregation of a basolateral marker requires cell-cell interactions

The time course of development of polarity of an apical (184-kD) and a basolateral (63-kD) plasma membrane protein of Madin-Darby canine kidney cells was followed using semiquantitative immunofluorescence on semithin (approximately 0.5-micron) frozen sections and monoclonal antibody probes. The 184-kD protein became highly polarized to the apical pole within the initial 24 h both in normal medium and in 1-5 microM Ca2+, which results in well-spread, dome-shaped cells, lacking tight junctions and other lateral membrane interactions. In contrast, the basolateral 63-kD membrane protein developed full polarity only after incubation in normal Ca2+ concentrations for greater than 72 h, a time much longer than that required for the formation of tight junctions (approximately 18 h) and failed to polarize in 1-5 microM Ca2+. These results demonstrate that intradomain restriction mechanisms independent of tight junctions, such as self-aggregation or specific interactions with the submembrane cytoskeleton, participate in the regionalization of at least some epithelial plasma membrane proteins. The full operation of these mechanisms depends on the presence of normal cell-cell interactions in the case of the basolateral 63-kD antigen but not in the case of the apical 184-kD protein.

Detergent sensitivity and splitting of isolated liver gap junctions

Isolated rat liver gap junctions were split by two methods. In the first method, isolated gap junctions were stabilized by cross-linking their cytoplasmic surfaces with glutaraldehyde under conditions that prevented the entry of glutaraldehyde into the "gap" region. The "stabilized" junctions were then split in the junctional gap with SDS. In the second procedure, unfixed gap junctions were split by incubation in urea-containing solutions. Junctional splitting was monitored by electron microscopy of thin sectioned and freeze fractured membrane pellets. Sidedness of the split junctional membranes was defined by labeling their cytoplasmic surfaces with glutaraldehyde-activated ferritin before splitting with urea. Gap junctional splitting did not result in any loss of protein components as determined by SDS-gel electrophoresis. The glutaraldehyde cross-linking procedure was also used to determine the effects of various detergents on the protein-protein interactions in the "gap" region. Of the detergents tested, only SDS caused junctional splitting.

Effect of temperature on the occluding junctions of monolayers of epithelioid cells (MDCK)

In previous works it was demonstrated that the monolayer of MDCK cells behaves as a leaky epithelium where the electrical resistance across reflects the sealing capacity of the occluding junction. In the present work we study whether this sealing capacity can be modified by temperature and whether this is accompanied by changes in the structure of the occluding junction. Monolayers were prepared on disks of nylon cloth coated with collagen and mounted as a flat sheet between two Lucite chambers. The changes in resistance elicited by temperature were large (306% between 3 and 37 degrees C), fast (less than 2 sec), and reversible. An Arrhenius plot of conductance versus the inverse of temperature shows a broken curve (between 22 and 31 degrees C), and the activation energies calculated (3.2 and 4.0 kcal X mol-1) fall within the expected values for processes of simple diffusion. The morphology of the occluding junction was evaluated in freeze-fracture replicas by counting the number of strands and the width of the band occupied by the junction every 133 nm. In spite of the change by 306% of the electrical resistance and the phase transition, we were unable to detect any appreciable modification of the morphology of the occluding junction. Since the freeze-fracture replicas also show a density of intramembrane particles (IMP) different in the apical from that in the basolateral regions of the plasma membrane, as well as differences between face E and face P, we also investigated whether this is modified by temperature. Cold increases the population of IMP, but does not affect their polarization with the incubation time it takes to elicit changes in electrical resistance.

(Na+ + K+)-ATPase correlated with a major group of intramembrane particles in freeze-fracture replicas of cultured chick myotubes

Immunofluorescence microscopy with a fluorescein-labeled monoclonal antibody was used to map the distribution of sodium- and potassium-ion stimulated ATPase [( Na,K]-ATPase) on the surface of tissue-cultured chick skeletal muscle. At this level of resolution it appeared that the (Na,K)-ATPase molecules were distributed nearly uniformly over the plasma membrane. These molecules could be cross-linked by use of the monoclonal antibody followed by a second antibody directed against the monoclonal antibody; the resulting fluorescent pattern was a set of small dots (patches) on the muscle surface. This pattern was stable over several hours, and there was little evidence of interiorization or of coalescence of the patches. Myotubes labeled with immunofluorescence were fixed in glutaraldehyde, cryoprotected with glycerin, then fractured and replicated by standard methods. Replicas of the immunofluorescence-labeled myotubes revealed clusters of intramembrane particles (IMP) only when the immunofluorescent images indicated a patching of the (Na,K)-ATPase molecules. Double antibody cross-linking of antigenic sites on myotubes with each of three other monoclonal antibodies to plasma membrane antigens likewise resulted in patched patterns of immunofluorescence, but in none of these cases were clusters of intramembrane particles found in freeze-fracture replicas. In each case it was shown that the (Na,K)-ATPase molecules were not patched. Other control experiments showed that patching of (Na,K)-ATPase molecules did not cause co-patching of one of the other plasma membrane proteins defined by a monoclonal antibody and did not cause detectable co-clustering of acetylcholine receptors. Detailed mapping showed that there was a one-to-one correspondence between immunofluorescent patches related to the (Na,K)-ATPase and clusters of IMP in a freeze-fracture replica of the same cell. We conclude that the intramembrane particles patched by double antibody cross-linkage of the (Na,K)-ATPase are caused by (Na,K)-ATPase molecules in the fracture plane. Quantification of the IMP indicated that the (Na,K)-ATPase-related particles account for up to 50% of particles evident in the replicas, or up to about 400 particles/micrometers2 of plasma membrane. Particles related to the (Na,K)-ATPase were similar to the average particle size and were as heterodisperse in size as the total population of IMP.(ABSTRACT TRUNCATED AT 400 WORDS)

Freeze-fracture studies on barley plastid membranes. VI. Location of the P700-chlorophyll a-protein 1

The photosystem I mutant of barley, viridis-zb63, which totally lacks the P700-chlorophyll a-protein 1 was characterised by rotary shadowed, freeze-fracture electron microscopy. Objective measurements were made of particle density and size distribution for all four fracture faces, and compared with values for wild-type. A highly significant reduction in the size of the PFu particles was found, which could be attributed to the loss of a population of large particles from the mutant PFu face. A corresponding loss of EFu pits was also observed. It is concluded that the photosystem I reaction centre and two or three ancillary polypeptides are located in the large PFu particles which account for two-thirds of the total, and that these particles span the membrane. Since no differences were seen on the PFs and EFs faces, there was no evidence for the localisation of any photosystem I in appressed granal membranes, supporting the concept of extreme lateral heterogeneity. A model is presented of the localisation of different functional polypeptide units to different freeze-fracture particles within the membrane. A peculiar feature of viridis-zb63 thylakoids was the presence of EFs particle arrays in vivo.

Monolayer freeze-fracture autoradiography: origins and directions

Monolayer freeze-fracture autoradiography (MONOFARG) is a product of two earlier methods: freeze-fracture autoradiography (FARG) and cell monolayer freeze-fracture. MONOFARG incorporates many of the basic principles and cytochemical goals of FARG while exploiting the technical advantages of monolayer freeze-fracturing. The latter method offers the opportunity to process freeze-dried 'half' membranes at room temperature. Although the feasibility of MONOFARG has been shown for qualitative analyses of split membranes, it's quantitative feasibility for transmembrane and in-plane analyses has just begun to be documented. An example of one aspect of that documentation is included in this report. The distribution of 125I-FITC-Concanavalin-A in the plane of split plasma membranes, human erythrocyte extracellular fracture faces, was examined and found to be homogeneous. The relevance of this finding to recently described double labelled membrane splitting experiments is discussed. The future of MONOFARG appears promising, especially in the application of the technique to biologically significant questions.

Spectroscopic assays for measuring quantities of erythrocyte membrane "halves"

The quantities of outer and inner"halves" produced by freeze-fracturing human erythrocyte membranes have been measured by visible and fluorescence spectroscopy. Assays have been developed that are based on the use of two membrane surface markers: hemoglobin (Hb), a native marker for the cytoplasmic side of the membrane, and fluoresceinated concanavalin A (FITC-Con-A), a marker for the extracellular side. Hb absorbance is proportional to the fraction of cytoplasmic "half" membranes, and FITC fluorescence is proportional to the fraction of extracellular "halves." A procedure is described for the preparation of surface-labeled, intact erythrocytes suitable for the formation of homogeneous, planar cell monolayers of square-centimeter dimensions on polylysine-treated glass (PL-glass). Cell monolayers were frozen and fractured, and the fractions of absorbance and fluorescence in each of the two split portions determined. The PL-glass portion of membrane contained a substantially higher ratio of fluorescence to absorbance than unsplit controls, and its paired portion, a complementary lower ratio, demonstrating that the PL-glass portion was significantly enriched in extracellular "half" membrane. Experiments investigating split membrane recovery show that the double labeled membrane splitting technique is well suited to analysis of the transmembrane distribution of membrane lipids and polypeptides using methods that do not require quantitation by electron microscopy.

Intramembrane particles and the organization of lymphocyte membrane proteins

An experimental system was developed in which the majority of all lymphocyte cell-surface proteins, regardless of antigenic specificity, could be cross-linked and redistributed in the membrane to determine whether this would induce a corresponding redistribution of intramembrane particles (IMP). Mouse spleen cells were treated with P-diazoniumphenyl- beta-D-lactoside (lac) to modify all exposed cell-surface proteins. Extensive azo- coupling was achieved without significantly reducing cell viability or compromising cellular function in mitogen- or antigen-stimulated cultures. When the lac-modified cell- surface proteins were capped with a sandwich of rabbit antilactoside antibody and fluorescein-goat anti-rabbit Ig, freeze-fracture preparations obtained from these cells revealed no obvious redistribution of IMP on the majority of fracture faces. However, detailed analysis showed a statistically significant 35 percent decrease (P less than 0.01) in average IMP density in the E face of the lac-capped spleen cells compared with control cells, whereas a few E-face micrographs showed intense IMP aggregation. In contrast, there was no significant alteration of P-face IMP densities or distribution. Apparently, the majority of E-face IMP and virtually all P-face IMP densities or distribution. Apparently, the majority of E-face IMP and virtually all P-face IMP do not present accessible antigenic sites on the lymphocyte surface and do not associate in a stable manner with surface protein antigens. This finding suggests that IMP, as observed in freeze-fracture analysis, may not comprise a representative reflection of lymphocyte transmembrane protein molecules and complexes because other evidence establishes: (a) that at least some common lymphocyte surface antigens are indeed exposed portions of transmembrane proteins and (b) that the aggregation of molecules of any surface antigen results in altered organization of contractile proteins at the cytoplasmic face of the membrane.

Specializations in filopodial membranes at points of attachment to the substrate

A mouse cell line (LM), which grows predominantly as spindle-shaped cells with numerous filopodia, was employed in this study. These filopodial projections appear to be important as sites of attachment to the substratum in LM cells. Morphologically the filopodia are slender projections from the cell body which usually attach to the substrate at their distal ends (filopodial footpads). Freeze-fracture of monolayer cultures in situ preserves the spatial relationship of filopodial processes to that of the cell body. Examination of these freeze-fracture preparations reveals a striking difference in the density of intramembrane particles (IMP) in the filopodial-footpad plasmalemma compared with the plasmalemma of the cell body (number of IMP in footpad > cell body). Additionally, there is a marked difference in the number of filipin-sterol complexes on the cell body, compared with the filopodial footpad, implying a difference in the cholesterol content in these regions (filipin-sterol complexes in footpad < cell body). These data suggest a structural and functional specialization of the filopodial-footpad plasma membrane which may be related to cell adhesion.

Qualitative and quantitative freeze-fracture studies on olfactory and nasal respiratory epithelial surfaces of frog, ox, rat, and dog. II. Cell apices, cilia, and microvilli

The densities and diameters of intramembranous particles in olfactory and nasal respiratory structures of frog, ox, rat and dog have been compared using the freeze-fracture technique. Dendritic endings and the various segments of the cilia of the olfactory receptor cells of a given species have identical particle densities (700--1,800 particles/micrometers2 in P- and 100--600 in E-faces). Densities in P-faces of respiratory cilia are about 1/3 of those in the olfactory cilia. E-face particle densities of these respiratory cilia are often higher than P-face densities. Microvillus P-face densities range from 700--2,000 (respiratory cell microvilli) to 1,800--3,400 particles/micrometers2 (olfactory supporting and Bowman's gland microvilli). Microvillus E-faces show no conspicuous mutual differences. Literature comparisons showed that odour concentrations at threshold are considerably lower (10(5)--10(10) times) than the concentrations of olfactory receptor ending intramembranous particles (5 microM--30 microM) expressed in the same units. Relative differences in particle distributions of the various cell structures studied are usually species-independent. Absolute values vary considerably with the species. Relative P-face particle densities of the supporting cell microvilli tend to correlate with those of dendritic ending structures. Particle diameters are usually similar to corresponding structures and fracture faces in the four species. Apical structures of supporting and Bowman's gland cells in rat and dog show rod-shaped particle aggregates in the P- and pits in their E-faces. Neither sex-dependency nor an influence related to physiological treatments on the particle distributions could be demonstrated.

Analysis of the structure of intramembrane particles of the mammalian urinary bladder

The luminal and discoid vacuole membranes of the superficial cell layer of the transitional epithelium of the mammalian urinary bladder have been studied by thin-sectioning and freeze-fracture-etch (FFE) electron microscope methods. For the FFE studies membranes were deposited on a cationized glass surface, covered by a thin copper disc, and fractured under liquid N2. Specimens were etched at -100 degrees C and replicated at -190 degrees C. A model of the lattice membrane derived from thin sections was used to predict the heights of the fracture faces above the glass surface. A hexagonal pattern of globular intramembrane particles spaced 160 A apart was seen in the external fracture (EF) face plaques as previously described and regarded as the dominant structure. However, very extensive areas of another pattern, seen before in only limited areas, have beeen found in the EF faces. The pattern consists of a smooth hexagonal lattice with the same space constant as the globular one but a different structure. By image analysis it consists of overlapping domains bordered by shared but incomplete metal rims. Each domain has a central spot of metal encircled by a shadow. The surface of the smooth lattice is partly complementary to the corresponding protoplasmic fracture (PF) face which shows a similar hexagonal lattice with the same space constant. The height of the smooth EF lattice above the glass substrate is the same as the plane of the center of the lipid bilayer predicted by the model. The mean heights of the particles of the globular EF lattice are greater than the total thickness of the membrane as predicted by the model and confirmed by measurements. The globular EF lattice is not complementary and it is concluded that the globular particles do not exist in the native membrane but arise artifactually during the preparatory procedures.

Surface-induced lamellar orientation of multilayer membrane arrays. Theoretical analysis and a new method with application to purple membrane fragments

The orientation of membrane fragments into a lamellar array by a flat surface is analyzed. This analysis includes processes such as centrifugation and drying and physical effects due to membrane fragment steric interactions, finite size, elasticity, and thermal fluctuations. Several model calculations of optimal orientational order in multilayer membrane arrays are presented. The predictions of a smectic A model agree quantitatively with the measured spatial dependence of the fluctuations in layer orientation in a multilamellar arrays. A new technique, based in part on this analysis, for the preparation of well-oriented multilamellar arrays of natural and artificial membranes, isopotential spin-dry centrifugation, is described. The method involves the use of specially designed inserts for the buckets of a standard vacuum ultracentrifuge. The membrane fragments to be oriented are sedimented from solution or suspension onto a substrate of a convenient material which forms a gravitational isopotential surface at high g. Sedimentation is accompanied by removal of the suspending medium at high g to produce oriented films with a selected degree of solvation. In addition, a method is described whereby small solute molecules can be maintained in constant concentration with the membrane fragments during this process. Initial application of the method to the orientation of purple membrane fragments is described. The degree of orientation obtained in this system is evaluated using freeze-fracture and scanning electron microscopy, optical birefringence, linear dichroism, and microscopy.

Linear arrays of intramembrane particles on microvilli in primate large intestine

Distinctive linear arrays of intramembrane particles were present in microvillar membranes of approximately 5% of surface columnar cells observed in freeze-fracture replicas of monkey colon and human rectum. On these cells, longitudinally-oriented rows of P face particles and corresponding E face grooves appeared on all exposed microvilli. The constituent particles varied from round (8-9 nm in diameter) to rod-shaped (18 nm long). Microvilli of the great majority of columnar cells displayed randomly distributed P face particles similar to those previously observed in small and large intestine of birds and small mammals. The significance of the linear arrays is not known. It is postulated that they may represent protein assemblies which are specific to a functionally-distinct subpopulation of primate intestinal columnar cells.