C-CAM (cell-CAM 105) is an adhesive cell surface glycoprotein with homophilic binding properties

Abstract

C-CAM (Cell-CAM 105) is a cell surface glycoprotein that is involved in cell-cell adhesion of rat hepatocytes in vitro. To elucidate the adhesion mechanism the binding properties of purified C-CAM were investigated. Using proteins immobilized on nitrocellulose it was found that radiolabeled C-CAM bound to C-CAM but not to a variety of other proteins. Partitioning in Triton X-114 showed that C-CAM has hydrophobic properties. In accordance with this, C-CAM was effectively incorporated into phosphatidylcholine liposomes by dialysis from octylglucoside-containing solutions. The C-CAM-containing liposomes bound specifically to isolated hepatocytes. This binding was blocked by Fab fragments of anti-C-CAM antibodies. Furthermore, preincubation of hepatocytes with anti-C-CAM antibodies followed by washing of the cells blocked binding of C-CAM-containing liposomes. At increasing C-CAM contents in the reconstituted liposomes a marked self-aggregation of the liposomes occurred. This aggregation was blocked by Fab fragments of anti-C-CAM antibodies and by alkaline pH. After neutralization a rapid reaggregation occurred. Neither C-CAM binding to C-CAM immobilized on nitrocellulose nor C-CAM-liposome aggregation required calcium ions. Liposomes reconstituted with C-CAM-depleted membrane glycoproteins did not self-aggregate or bind to hepatocytes. Thus, it is concluded that C-CAM can bind specifically to C-CAM in a homophilic binding reaction that does not require calcium. Accordingly, C-CAM has the potential of directly mediating cell-cell adhesion via C-CAM-C-CAM binding between adjacent cells.