Homophilic intercellular adhesion mediated by C-CAM is due to a domain 1-domain 1 reciprocal binding

The cell adhesion molecule C-CAM belongs to the immunoglobulin superfamily and is expressed in epithelia, vessel endothelia, and hematopoietic cells. Differential splicing gives rise to different isoforms, of which the major two are C-CAM1 and C-CAM2, which both have four Ig-like domains in their extracellular portions, but differ in their cytoplasmic domains. Two different allelic variants of C-CAM, named a and b, occur in the rat. The adhesive binding mechanism(s) of C-CAM is not known in detail. Evidence for both homophilic and heterophilic binding has been presented, and different species and splice variants of C-CAM have shown differences in temperature and cation dependence when expressed in different cell types. Here, we have analyzed the binding mechanism of rat C-CAM2a that was expressed in CHO cells. In this system C-CAM2a-mediated adhesion was calcium- and temperature-independent. C-CAM2a-transfected cells did not adhere to nontransfected cells, demonstrating that the binding was homophilic. Cells transfected with C-CAM2a in which the N-terminal Ig-domain (D1) was deleted did not aggregate, and cells with intact C-CAM2a could not bind to these cells. This was in contrast to cells that were transfected with C-CAM2a in which the fourth Ig-like domain (D4) had been deleted; they both aggregated and bound to cells with intact C-CAM2a. Thus, C-CAM2a mediates intercellular adhesion of CHO cells by a homophilic mechanism, in which the D1 domain binds reciprocally to a D1 domain on an opposed C-CAM molecule.