Structural basis for PECAM-1 homophilic binding

Abstract

Platelet endothelial cell adhesionmolecule-1 (PECAM-1) is a 130-kDamember of the immunoglobulingenesuperfamily (IgSF) that ispresentonthesurfaceof circulatingplateletsand leukocytes, and highly expressed at the junctions of confluent endothelial cellmonolayers. PECAM-1-mediated homophilic interactions, known to bemediated by its 2amino-terminal immunoglobulin homology domains, are essential for concentrating PECAM-1 at endothelial cell intercellular junctions, where it functions to facilitate diapedesis, maintain vascular integrity, and transmit survival signals into the cell. Given the importance of PECAM-1- mediated homophilic interactions inmediating each of these cell physiological events, and to reveal the nature and orientation of the PECAM-1-PECAM-1 homophilic-binding interface, we undertook studies aimed at determining the crystal structure of the PECAM-1 homophilic-binding domain, which is composed of amino-terminal immunoglobulin homology domains 1 and 2 (IgD1 and IgD2). The crystal structure revealed that both IgD1 and IgD2 exhibit a classical IgSF fold, having a β-sandwich topology formed by 2 sheets of antiparallelbstrands stabilized bythehallmark disulfidebond between theBandF strands. Interestingly, despite previous assignment to the C2 class of immunoglobulin-like domains, the structure of IgD1 reveals that it actually belongs to the I2 set of IgSF folds. Both IgD1 and IgD2 participate importantly in the formation of the trans homophilic-binding interface, with a total buried interface area of >2300 Å2. These and other unique structural features of PECAM-1 allow for the development of an atomic-level model of the interactions that PECAM-1 forms during assembly of endothelial cell intercellular junctions.