Electrophoretic analysis of the major polypeptides of the human erythrocyte membrane.

A fluid mosaic model is presented for the gross organization and structure of the proteins and lipids of biological membranes. The model is consistent with the restrictions imposed by thermodynamics. In this model, the proteins that are integral to the membrane are a heterogeneous set of globular molecules, each arranged in an amphipathic structure, that is, with the ionic and highly polar groups protruding from the membrane into the aqueous phase, and the nonpolar groups largely buried in the hydrophobic interior of the membrane. These globular molecules are partially embedded in a matrix of phospholipid. The bulk of the phospholipid is organized as a discontinuous, fluid bilayer, although a small fraction of the lipid may interact specifically with the membrane proteins. The fluid mosaic structure is therefore formally analogous to a two-dimensional oriented solution of integral proteins (or lipoproteins) in the viscous phospholipid bilayer solvent. Recent experiments with a wide variety of techniqes and several different membrane systems are described, all of which abet consistent with, and add much detail to, the fluid mosaic model. It therefore seems appropriate to suggest possible mechanisms for various membrane functions and membrane-mediated phenomena in the light of the model. As examples, experimentally testable mechanisms are suggested for cell surface changes in malignant transformation, and for cooperative effects exhibited in the interactions of membranes with some specific ligands. Note added in proof: Since this article was written, we have obtained electron microscopic evidence (69) that the concanavalin A binding sites on the membranes of SV40 virus-transformed mouse fibroblasts (3T3 cells) are more clustered than the sites on the membranes of normal cells, as predicted by the hypothesis represented in Fig. 7B. T-here has also appeared a study by Taylor et al. (70) showing the remarkable effects produced on lymphocytes by the addition of antibodies directed to their surface immunoglobulin molecules. The antibodies induce a redistribution and pinocytosis of these surface immunoglobulins, so that within about 30 minutes at 37 degrees C the surface immunoglobulins are completely swept out of the membrane. These effects do not occur, however, if the bivalent antibodies are replaced by their univalent Fab fragments or if the antibody experiments are carried out at 0 degrees C instead of 37 degrees C. These and related results strongly indicate that the bivalent antibodies produce an aggregation of the surface immunoglobulin molecules in the plane of the membrane, which can occur only if the immunoglobulin molecules are free to diffuse in the membrane. This aggregation then appears to trigger off the pinocytosis of the membrane components by some unknown mechanism. Such membrane transformations may be of crucial importance in the induction of an antibody response to an antigen, as well as iv other processes of cell differentiation.

The fluid mosaic model of the structure of cell membranes.

A surface glycoprotein modulating drug permeability in Chinese hamster ovary cell mutants.

Solubilization of membranes by detergents

THE FLUID MOSAIC MODEL OF THE STRUCTURE OF CELL MEMBRANES Reprinted with permission from Science, Copyright AAA, 18 February 1972, Volume 175, pp. 720–731.

Chemical probes of extended biological structures: Synthesis and properties of the cleavable protein cross-linking reagent [35S]dithiobis(succinimidyl propionate)

Disposition of the major proteins in the isolated erythrocyte membrane. Proteolytic dissection.

Phosphorylation of endogenous substrates by erythrocyte membrane protein kinases. I. A monovalent cation-stimulated reaction.

Surface proteins of ram spermatozoa were studied at three stages of post-testicular develop ment: immediately after transport out of the testis; after epididymal transit; and after exposure to accessory sex gland secretions. Testicular and cauda epididymal spermatozoa, together with the fluids which normally surround them, were collected from conscious rams through catheters inserted into the rete testis and vas deferens, respectively. Ejaculated spermatozoa were obtained by electrical stimulation. The washed, viable spermatozoa were radioiodinated by the surface selective hactoperoxidase-glucose oxidase technique. Total sodium dodecyl sulfate-soluble cellular proteins were fractionated by one- and two-dimensional sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Labeled surface components were identified and distinguished from unlabeled cellular proteins by autoradiography of dried gel slices. The ehectrophoretic analysis reveals that the predominant radioiodinated surface components of testicular spermatozoa are seven polypeptides that migrate with apparent molecular weights of 78,000â€”132,000. Spermatozoa collected from the cauda epididymidis lack the major components characteristic of testicular spermatozoa, but have other unique surface features; these include two large pohypeptides (molecular weights, 71,000 and 116,000 daltons) and several smaller com ponents. Most prominent among the hatter is a diffuse band of apparent molecular weight 24,000. These striking changes in accessible surface proteins take place during epididymal transit without alterations in the electrophoretic pattern of total cellular proteins. Further surface renovation after spermatozoal maturation is minimal, as shown by the similarity in the radioiodination pattern of ejaculated and cauda epididymal spermatozoa. Since extracehlular fluids may play a role in surf;ce modifications of spermatozoa in the epi didymis, testicular spermatozoa were incubated in the presence of radioiodinated proteins of rete testis and cauda epididymal fluid. Uptake of labeled protein from rete testis fluid is nonspecific, but adsorption of cauda epididymal fluid proteins by testicular spermatozoa is highly selective. The major adsorbed component migrates with an apparent molecular weight of 24,000 and appears to correspond to the major surface component detectable by radioiodination of cauda epididymal spermatozoa. The anomalous migration of this protein on two-dimensional gel suggests that it is highly glycosylated. These results provide direct biochemical evidence that spermatozoa undergo extensive surface modifications during passage through the epididymis where they become almost totally dependent on exogenous substrates for survival and develop the capacity for coordinated movement. Adsorption of extracelhuhar proteins from the luminal fluids of the testis and epi didymis may contribute to these critical functions of the fully differentiated cell.

Grant Fairbanks

Papers

Electrophoretic analysis of the major polypeptides of the human erythrocyte membrane.

Chemical probes of extended biological structures: Synthesis and properties of the cleavable protein cross-linking reagent [35S]dithiobis(succinimidyl propionate)

Disposition of the major proteins in the isolated erythrocyte membrane. Proteolytic dissection.

Phosphorylation of endogenous substrates by erythrocyte membrane protein kinases. I. A monovalent cation-stimulated reaction.

Post-testicular developmental changes in the ram sperm cell surface and their relationship to luminal fluid proteins of the reproductive tract.