Expression of human PECAM-1 isoforms by REN cells. (A) Schematic representation of full-length PECAM-1 and different truncated and point mutant isoforms. TD indicates transmembrane domain; CD, cytoplasmic domains; RhP, full-length human PECAM-1. The 6 Ig-like extracellular domain, equal for all constructs, are not shown. ΔCD is a construct lacking the entire PECAM-1 cytoplasmic domain. Δ11-16 and Δ15-16 are truncated constructs in which these specific exons have been deleted. Specific point mutations are shown in exons 13 and 14 for Y663F and Y686F isoforms. (B) Immunobloting of full-length wild-type and mutant PECAM-1 isoforms expressed in REN cells. Proteins were extracted from transfected and nontransfected REN cell lysates and immunoblotted with monoclonal anti-PECAM 4G6 to human PECAM-1. Beta-actin was used as a control for protein loading. Protein molecular weight markers (in kilodaltons) are shown on the left. Nontransfected REN cells do not express PECAM-1 (REN^−/−, lane 1). (C) Cell-surface expression of PECAM-1 was determined by FACS using mouse anti-human PECAM-1 monoclonal antibodies. Mean fluorescence intensity values were normalized to PECAM-1 expression level for RhP (n ≥ 3; *P < .01, **P < .001 to RhP).