PECAM-1/CD31 trans-homophilic binding at the intercellular junctions is independent of its cytoplasmic domain; Evidence for heterophilic interaction with integrin alpha v beta 3 in cis

PECAM-1/CD31 is a cell adhesion and signaling molecule that is enriched at the endothelial cell junctions. Alternative splicing generates multiple PEC AM-1 splice variants, which differ in their cytoplasmic domains. It has bee n suggested that the extracellular ligand-binding property, homophilic vers us heterophilic, of these isoforms is controlled by their cytoplasmic tails . To determine whether the cytoplasmic domains also regulate the cell surfa ce distribution of PECAM-1 splice variants, we examined the distribution of CD31-EGFPs (PECAM-1 isoforms tagged with the enhanced green fluorescent pr otein) in living Chinese hamster ovary cells and in PECAM-1-deficient endot helial cells. Our results indicate that the extracellular, rather than the cytoplasmic domain, directs PECAM-1 to the cell-cell borders. Furthermore, coculturing PECAM-1 expressing and deficient cells along with transfection of CD31-EGFP cDNAs into PECAM-1 deficient cells reveal that this PECAM-1 lo calization is mediated by homophilic interactions. Although the integrin al pha v beta 3 has been shown to interact with PECAM-1, this trans-heterophil ic interaction was not detected at the borders of endothelial cells. Howeve r, based on cocapping experiments performed on proT cells, we provide evide nce that the integrin alpha v beta 3 associates with PECAM-1 on the same ce ll surface as in a cis manner.