title: Receptor binding and internalization of immobilized transcobalamin II by mouse leukaemia cells.
abstract: Membrane transport of vitamin B12 (cyanocobalamin; Cbl) into mammalian cells is mediated by the serum protein transcobalamin II (TCII). In mouse leukaemia L1210 cells, TCII-Cbl binds to membrane receptors in a rapid, temperature-independent step and is internalized by a slow, temperature-dependent process. To delineate the location of receptors on these cells, we have constructed a visual probe by covalently coupling purified TCII-Cbl to submicrometre latex particles (minibeads). We report here that when L1210 cells are incubated with minibeads containing TCII-Cbl at 4 degrees C and examined by scanning electron microscopy (SEM), the particles are found attached predominantly to microvilli. Incubation of the cells at 37 degrees C results in the internalization of the minibeads. As visualized by transmission electron microscopy (TEM), this endocytotic process seems to occur in clathrin-coated pits and vesicles at the cell surface.
mesh_terms: D000818:Animals; D001798:Blood Proteins / Q000378:metabolism*; D004705:Endocytosis; D007939:Leukemia L1210; D051379:Mice; D008854:Microscopy, Electron; D008863:Microspheres; D008871:Microvilli / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism*; D013696:Temperature; D014155:Transcobalamins / Q000378:metabolism*
chemical_list: D001798:Blood Proteins; D011956:Receptors, Cell Surface; D014155:Transcobalamins; C028176:transcobalamin receptor