Functional significance of platelet membrane glycoprotein p62 (GP VI), a putative collagen receptor

We previously suggested that platelet 62 kDa protein (p62, GP VI) is a putative collagen receptor by the studies on platelets and antiplatelet antibody in a patient (YA) with idiopathic thrombocytopenic purpura (ITP) who showed defective collagen-induced platelet function. Herein, we confirm that p62/p57 (reduced/unreduced conditions) is a specific membrane protein recognized by YA antibody using an improved immunoprecipitation technique. To further study the functional role of this protein, a crude p57 fraction ('p57') was obtained from human platelets by serial chromatography including DEAE-Sepharose CL-6B and WGA-agarose followed by preparative sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). This 'p57' diminished the platelet-aggregating activity of collagen and of YA immunoglobulin G (IgG). N-terminal amino acid sequence analysis of 'p57' showed that it contained intercellular adhesion molecule 2 (ICAM-2). Immunoprecipitation experiments using platelet lysates and anti-ICAM-2 monoclonal antibody demonstrated that ICAM-2 molecules had a molecular weight similar to p62 on SDS-PAGE. However, ICAM-2 was also expressed on YA's p62-deficient platelets both on immunoprecipitation and flow cytometric analysis. Thus, ICAM-2 seems to be different from p62.