Qh. Sun et al., INDIVIDUALLY DISTINCT IG HOMOLOGY DOMAINS IN PECAM-1 REGULATE HEMOPHILIC BINDING AND MODULATE RECEPTOR AFFINITY, The Journal of biological chemistry, 271(19), 1996, pp. 11090-11098
PECAM-1 (CD31) is a 130-kDa member of the immunoglobulin (Ig) gene sup
erfamily that is constitutively expressed at high concentration at end
othelial cell intercellular junctions and at moderate density on the s
urface of circulating leukocytes and platelets, Recent in vitro and in
vivo studies have shown that PECAM-1 plays a central, role in mediati
ng the extravasation of leukocytes from the vessel wall in response to
inflammatory mediators. To study the binding characteristics of PECAM
-1, phospholipid vesicles were prepared and examined by flow cytometry
and immunofluorescence microscopy for their ability to associate with
each other and with cells. Proteoliposomes containing high concentrat
ions of PECAM-1 interacted homophilically with each other, forming lar
ge self-aggregates. PECAM-1 proteoliposomes, as well as soluble bivale
nt PECAM-1 in the form of a PECAM-1/IgG immunoadhesin, associated homo
philically with cells expressing human, but not murine, PECAM-1. This
binding could be completely inhibited by monoclonal antibody Fab fragm
ents specific for Ig homology Domain 1 or Domains 1 + 2. Binding studi
es using cells expressing human PECAM-1 deletion mutants and murine/hu
man chimeras confirmed that both Ig Domains 1 and 2 were both necessar
y and sufficient for hemophilic binding. In contrast, engagement of me
mbrane-proximal Domain 6 with monoclonal antibody Fab fragments had th
e opposite effect and augmented the binding of PECAM-1 proteoliposomes
to cells. Thus, PECAM-1, like certain integrins, appears to be capabl
e of antibody-induced conformational changes that alter affinity for i
ts ligand. Similar changes induced by physiologic stimuli could be imp
ortant in regulating the function of PECAM-1 in vascular cells.