Garth L. Nicolson

The Fluid Mosaic Model of the Structure of Cell Membranes

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�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�C instead of 37�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 interaction of Ricinus communis agglutinin with normal and tumor cell surfaces

Abstract Two plant agglutinins were isolated and purified from Ricinus communis beans by affinity chromatography. Both of the agglutinins were inhibited by sugars containing terminal β- d -galactose-like residues, but the smaller agglutinin (mol. wt approximately 60 000) was additionally inhibited by N- acetyl- d -galactosamine-like sugars. The larger agglutinin (mol. wt approximately 120 000) was used to specifically agglutinate several normal and transformed cell lines. The agglutinin consistently agglutinated transformed cells at much lower concentrations than those required to agglutinate normal cell lines unless the normal cells were first treated with low concentrations of trypsin. The significance of the change in agglutinability of transformed or trypsinized cells is discussed in relation to the topology of the cell surface.

Freeze-etch localization of concanavalin A receptors to the membrane intercalated particles of human erythrocyte ghost membranes

Freeze-fracture, freeze-etch and molecular labeling techniques localize concanavalin A receptors to the membrane intercalated particles of human erythrocyte ghost membranes. The concanavalin A receptor is a Band III component [4]. We propose that the membrane intercalated particle represents an oligomeric structure containing, at least, the principal integral proteins [glycophorin and Band III component(s) interacting with the protein spectrin at the membrane inner surface. Previous work implies Band III component(s), integral membrane proteins and the membrane intercalated particles in a variety of transmembrane phenomena. We hypothesize that the membrane intercalated particles represent an amphipatic structure (“permeaphore.”∗) which topologically and structurally interrupts hydrophobic bilayer membrane domains.

Mobility and the Restriction of Mobility of Plasma Membrane Lectin-Binding Components

Labeling by ferritin-conjugated agglutinins from Ricinus communis was used to demonstrate the relative mobilities of the agglutinin receptors located in specific regions on plasma membranes of rabbit spermatozoa. The relative mobility of lectin receptors was higher on postacrosomal regions of sperm than on acrosomal and tail regions. Lectin-induced clustering could not be demonstrated in the acrosomal and tail regions, an indication of the existence of localized restraints on the mobilities of lectin receptors. A system of transmembrane restraints may maintain the segregation of plasma membrane components into membrane domains on certain highly differentiated cells.

Terminal Saccharides on Sperm Plasma Membranes: Identification by Specific Agglutinins

Six specific agglutinins were used to identify the terminal sugar residues in the surface oligosaccharides of rabbit and hamster spermatozoa by specific agglutination. Species differences in epididymal sperm were found in the terminal residues, resembling α-D-mannose, D-galactose, N-acetyl-D-glucosamine, and N-acetyl-D-galactosamine. Species similarities were found in terminal residues, resembling L-fucose and N-acetylneuraminic acid. When ejaculated rabbit sperm were compared to epididyimal sperm, the latter were more agglutinable with a specific agglutinin recognizing N-acetyl-D-glucosamine.

Inhibition of hamster fertilization by phytoagglutinins

Abstract Unfertilized eggs of the golden hamster were treated with phytoagglutinins and inseminated in vitro with capacitated spermatozoa. Ricinus communis agglutinin was most effective in preventing fertilization, followed by wheat germ agglutinin, Dolichos biflorus agglutinin and finally concanavalin A. The agglutinin-mediated block to fertilization is related to the saccharide-binding activities of agglutinins, because inclusion of the appropriate saccharide inhibitor counteracted the actions of the agglutinins. It is proposed that the agglutinins bind to specific oligosaccharides of the zona pellucida and induce extensive cross-linking of adjacent saccharide chains in such a way that sperm-born zona lysins can no longer depolymerize the zona material. Spermatozoa failed to bind to zona surfaces following treatment of eggs with high concentrations of wheat germ agglutinin, but not with the other agglutinins, suggesting that N -acetyl- d -glucosamine-like or N -acetylneuraminic acid-like residues may be essential or sterically close to sperm attachment sites on the zona pellucida of the hamster egg.

Outer membrane terminal saccharides of bovine liver nuclei and mitochondria

Abstract Specific agglutinins were used to identify terminal residues on membrane-bound oligosaccharides of purified bovine nuclei and mitochondria. Saccharides found on nuclear outer membranes were: α- d -mannose-like, d -galactose-like, N -acetyl- d -glucosamine-like, N -acetyl- d -galactosamine-like, and l -fucose-like residues. These same saccharides, with the exception of l -fucose-like and possibly N -acetyl- d -galactosamine-like residues, were found on mitochondrial outer membranes.

Concanavalin A as A Quantitative and Ultrastructural Probe for Normal and Neoplastic Cell Surfaces

Concanavalin A (Con A) has been popularly used as a cell surface probe for normal and neoplastic cells. Differences between normal fibroblasts and their transformed derivatives were examined using Con A agglutination, quantitative labeling with 125I-Con A and ultrastructural labeling with fluorescent- or ferritin-Con A. Con A agglutinates confluent-SV3T3 and 3T3 cells at midpoints of 20-60 and greater than, 500-2,000 mug/ml, respectively, and sparse cells at 5-15 and 1,200-1,500 mg/ml, respectively. Quantitative binding of 125I-Con A at 4 degrees C (10 or 15 min) with saturating lectin concentrations does not indicate a difference in the number of Con A receptors on sparse or confluent 3T3 and SV3T3 cells similar to many publications, but contrary to Noonan and Burger (1973). Under these conditions of labeling, ferritin-Con A is not internalized, indicating absence of endocytosis. The lateral mobility of Con A receptors and their relative ability to be aggregated on the cell surface by Con A was investigated with fluorescent- and ferritin-Con A. The initial distribution of Con A receptors on 3T3, SV3T3 and MSV3T3 cells under conditions of labeling at low temperature (0-5 degrees C) or to fixed cells was essentially randomly dispersed, but changes quickly to aggregated on SV3T3 and MSV3T3 (but not 3T3) after shifting the temperature to 20-37 degrees C, indicating, in general, a greater relative mobility of Con A receptors on SV3T3 and MSV3T3 cells. The aggregated Con A receptors seem to be directly involved in cell agglutination because they are usually found at the sites of cell-to-cell contact during 10 min agglutination experiments with ferritin-Con A. When confluent-3T3 cells are labeled on monolayer with ferritin-Con A at 0-4 degrees C, washed and then shifted to 20-37 degrees C for 10-15 min prior to in situ embedding, two classes of Con A receptors can be identified. One class appears to have low relative mobility and is associated with the 3T3 cells extensive subplasma membrane microfilament network, while the other is capable of being aggregated and eventually endocytosed. On confluent-SV3T3 cells, only the latter class of receptors appears to be present, indicating a possible loss of cytoplasmic control over the distribution and mobility of lectin-binding sites on transformed cell surfaces.

Invasion of Vascular Endothelium and Organ Tissue in Vitro by B16 Melanoma Variants

When cancer cells become blood-borne, they have the potential to form secondary tumor foci at distant sites. In a variety of cancers, as well as animal tumor models, blood-borne metastasis occurs often to particular secondary sites (1–4). Although most regional metastases can be explained strictly on anatomic circulatory pathways or on mechanical lodgment of blood-borne tumor emboli in the first capillary bed encountered, distant blood-borne organ colonization by metastatic tumor cells does not always follow this pattern. There are numerous examples where malignant tumors tend to metastasize to unusual secondary locations. For example, prostatic carcinoma often metastasizes to bone, small cell carcinoma of the lung spreads at high frequency to the brain, and neuroblastoma to adrenal glands and liver (1,2).

The Cellular Interactions of Metastatic Tumor Cells with Special Reference to Endothelial Cells and their Basal Lamina-Like Matrix

Major goals in the prevention of cancer deaths are understanding how malignant tumor cells spread from primary sites to establish near or distant metastases and preventing this process from occurring. Metastasis involves a complex sequence of events in which malignant cells invade the surrounding host tissues, penetrate into lymphatics and blood vessels, detach from the primary tumor mass and spread to other sites where they must implant, invade and finally proliferate to form new metastatic colonies (for reviews see 1–8), This sequence is discussed in the reviews cited above; the present article will be limited to a discussion of the final events involved in the formation of hematogenous metastases (8).