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Biomedical subjects

M S Bretscher

Publications and source records attributed to M S Bretscher.

At least 37 records · Page 2Linked to original sources
PubMed1985

Heparan sulphate proteoglycans and their polypeptide chains from BHK cells.

A simple procedure for partially purifying membrane-associated 35S-labelled proteoglycans from BHK cells is described. The labelled molecules are mainly heparan sulphate glycoproteins, and their average half-lives are approximately 3 h in culture. A new method, which depends on the degradation of heparan sulphate by nitrous acid at low pH, has been devised to determine the size of the polypeptide moiety of this class of molecule. The BHK cell heparan sulphate proteoglycans contain three main polypeptides having mol. wts. of approximately 65, 85 and 120 kd.

Animals↗
PubMed1985

The morphology of endosomes in giant HeLa cells.

The endosomal compartment of giant HeLa cells was labelled with a transferrin-horse radish peroxidase (HRP) conjugate. Serial thin sections from the leading lamella of a cell are presented; they show that the endosomal compartment contains a tubular system connected to vesicular structures. In addition, small (approximately 50 nm) coated vesicles are seen in the leading lamella.

Cell Membrane↗
PubMed1984

Endocytosis: relation to capping and cell locomotion.

Most mammalian cells, such as fibroblasts, continuously internalize part of their surface membrane by endocytosis, and then later return it to the cell surface. This cyclical process is initiated by coated pits in the plasma membrane. These pits collect specific receptors plus lipid for internalization, but exclude other proteins. On a motile cell, the sites of endocytosis (randomly located on the cell) and those of membrane return (located at the front of the cell) are not coincident. This causes a bulk flow of lipid plus receptors in the plasma membrane, away from the front of the cell. Large objects on the cell surface are swept to the rear of the cell by this flow, a process called capping. Cells may use this polarized endocytic cycle to move.

Antigens, Surface↗
PubMed1983

Distribution of ferritin receptors and coated pits on giant HeLa cells.

HeLa cells bind horse spleen ferritin when the two are incubated at 0 degrees C. Since the majority of this bound ferritin is located in coated pits, we conclude that the ferritin binds to a specific receptor which takes part in an endocytic cycle. When substrate-attached and well-spread giant HeLa cells are briefly labelled at 0 degrees C with ferritin, ferritin particles are found to be concentrated towards the cell periphery, where they exist largely outside coated pits. This peripheral concentration is a property of circulating (and not just newly synthesized) receptors because it is not affected by prior incubation of giant cells in cycloheximide. However, coated pits are themselves roughly uniformly distributed over the surface of these cells. These results provide evidence that the membrane internalised by coated pits on these cells is returned to the cell surface at the leading edge of the cell. Because of this separation of the sites of endocytosis and exocytosis, a flow of membrane must occur across the cell surface. This flow is composed of lipid plus receptors. The implications of this for capping and for cell spreading are discussed.

Biological Transport, Active↗
PubMed1983

Distribution of receptors for transferrin and low density lipoprotein on the surface of giant HeLa cells.

Giant HeLa cells, having a spread diameter of about 200 micrometers, were briefly surface-labeled at 0 degree C with 125I-labeled transferrin, low density lipoprotein, anti-HeLa cell antibody, or concanavalin A. The cells were washed at 0 degree C, fixed, and autoradiographed. The distribution of grains when either anti-HeLa cell antibodies or concanavalin A was used was roughly as expected: the cell surfaces appeared uniformly labeled. When either transferrin or low density lipoprotein was used, about half the labeled cells had a nonuniform distribution of grains. On round cells, the cell periphery was more densely labeled than the middle of the cell; on elongated cells, cell protrusions were often more highly labeled than the rest of the cell. The simplest interpretation of these results is that, during their endocytic cycles through these cells, the transferrin and low density lipoprotein receptors are returned to the cell surface at the cell's leading edge.

Autoradiography↗
PubMed1982

Endocytosis, the sorting problem and cell locomotion in fibroblasts.

Fibroblasts endocytose lipid plus a subset of plasma membrane proteins over their entire surface and reinsert this into the plasma membrane at the cell's leading edge. This process is used to extend the fibroblast forwards. This circulation causes a flow of these endocytosed molecules over the cell's surface. Molecules, such as proteins, sitting in this flow can distribute themselves randomly by Brownian motion, but large objects (or small tethered ones) cannot. These large objects therefore cap. A mechanism is presented whereby this process could be used for locomotion using many weak interactions with the substrate. In addition it is suggested that the observed selectivity of coated pits may be sufficient to sort out proteins during transfer of membrane from one organelle to another so that the specific characters of the parent membranes are maintained.

Animals↗
PubMed1982

Transferrin receptor and its recycling in HeLa cells.

The transferrin receptor is a 180 000-dalton protein which can be dissociated to two 90 000-dalton polypeptides under reducing conditions. It can be labelled by lactoperoxidase-catalysed iodination on the cell surface at 0 degree C. Trypsin digestion of labelled cells at 0 degree C can be used to degrade those receptors on the cell surface; they release a 70 000-dalton soluble fragment which binds to transferrin. When cells are labelled at 0 degree C, then warmed to 37 degrees C, the labelled receptors enter the cells and become trypsin resistant. These receptors enter the cells, probably via coated pits, with a half-life of approximately 5 min. Since there is about three times as much receptor inside cells as on the surface, this means that transit through the cell to the cell surface takes approximately 21 min, if all receptors are on the same cycling pathway.

Coated Pits, Cell-Membrane↗
PubMed1980

Coated pits act as molecular filters.

Two proteins--theta and H63 antigens--are found to be excluded from coated pits on fibroblast plasma membranes. Coated pits thus act as molecular filters, pinching off, into the cell, lipid vesicles containing a limited number of specific receptors but excluding other plasma membrane proteins. This constitutes part of a lipid flow cycle that was proposed earlier to explain the capping of crosslinked surface antigens.

Animals↗
PubMed1979

Capping of exogenous Forssman glycolipid on cells.

When motile cells are incubated with Forssman glycolipid, the antigen is incorporated into the cells' plasma membranes. If cross-linked by antibody, the patched glycolipids cap. This process is sensitive to those drugs that are known to inhibit capping of protein antigens. The results support a flow mechanism for capping.

Animals↗