INTEGRITY OF THE HOMOPHILIC BINDING-SITE IS REQUIRED FOR THE PREFERENTIAL LOCALIZATION OF NCAM IN INTERCELLULAR CONTACTS

The neural cell adhesion molecule NCAM is a member of the immunoglobul in (Ig) superfamily. NCAM can undergo hemophilic binding and heterophi lic interactions with cell surface components and is often concentrate d at sites of intercellular contact. To investigate the molecular basi s of this biased surface distribution, we examined L cell transfectant s expressing wildtype or mutant forms of chick NCAM-140 by laser scann ing confocal microscopy. Mutant NCAMs that lacked Ig-like domains 1, 2 , 4, or 5 were preferentially localized in contact regions. However, t he relative concentration of these mutant NCAMs in contact sites was s ubstantially reduced compared with wild-type NCAM. In contrast, NCAM r edistribution to intercellular contacts was abolished in cells express ing mutant NCAMs that either lacked Ig-like domain 3 or contained muta tions in the hemophilic binding site in this domain. In heterotypic co ntacts between PC12 cells and L cell transfectants, colocalization of rat NCAM and chick NCAM was again dependent on the integrity of the he mophilic binding site of the NCAM expressed on L cells. These results provide evidence that hemophilic binding is the main mechanism by whic h NCAM becomes redistributed to intercellular contacts. They also impl icate-a role for other Ig-like domains in the accumulation of NCAM at cell-cell contacts.