A-CAM: a 135-kD receptor of intercellular adherens junctions. I. Immunoelectron microscopic localization and biochemical studies.
The recently described adherens junction-specific 135-kD protein (Volk, T., and B. Geiger, 1984, EMBO (Eur. Mol. Biol. Organ.) J., 3:2249-2260) was localized along cardiac muscle intercalated discs by immunogold labeling of ultrathin frozen sections. Analysis of this labeling indicated that the 135-kD protein, adherens junction-specific cell adhesion molecule (A-CAM), is tightly associated with the plasma membrane unlike vinculin labeling, which was present along the membrane-bound plaques of the fascia adherens. In cultured chick lens cells, A-CAM was associated with Ca2+-dependent junctions that were cleaved upon a decrease of extracellular Ca2+ concentrations to less than or equal to 0.5 mM. In the chelator-separated junction, A-CAM became exposed to exogenously added antibodies or to proteolytic enzymes. Upon addition of trypsin to EGTA-treated cells, A-CAM was cleaved into three major cell-bound antigenic peptides with apparent molecular masses of 78, 60, and 46 kD, suggesting that the extracellular domain of A-CAM has a size greater than or equal to kD. Incubation of electrophoretic gels with 125I-concanavalin A (Con A) indicated that one of the major Con A-binding proteins in chicken lens membranes is a integral of 135-kD glycoprotein that was partially purified on Con A-Sepharose column and identified as A-CAM by immunoblotting. Detergent partitioning assay using Triton X-114 biphasic system was carried out to determine whether A-CAM displays properties of an integral membrane protein. This assay indicated that the intact A-CAM molecule was recovered in the buffer phase but its cell-associated tryptic peptides, which presumably lost a great part of the A-CAM extracellular extension, readily partitioned into the detergent phase. The results obtained in this and in the following paper (Volk, T., and B. Geiger, 1986, J. Cell Biol., 103:1451-1464) strongly suggest that A-CAM is a Ca2+-dependent adherens junction-specific membrane glycoprotein that is involved in intercellular adhesion in these sites.
D000818:Animals; D015703:Antigens, CD; D015820:Cadherins; D002118:Calcium / Q000494:pharmacology; D015815:Cell Adhesion Molecules; D002478:Cells, Cultured; D002645:Chickens; D007158:Immunologic Techniques; D007365:Intercellular Junctions / Q000032:analysis*; D007908:Lens, Crystalline / Q000032:analysis; D008565:Membrane Proteins / Q000032:analysis* / Q000276:immunology / Q000502:physiology; D008854:Microscopy, Electron; D010455:Peptides / Q000032:analysis
D015703:Antigens, CD; C050361:CDH2 protein, human; D015820:Cadherins; D015815:Cell Adhesion Molecules; D008565:Membrane Proteins; D010455:Peptides; D002118:Calcium