Structural identification of the hematopoietic progenitor antigen ER‐MP12 as the vascular endothelial adhesion molecule PECAM‐1 (CD31)

The monoclonal antibody ER-MP12 was recently described to recognize an antigen present on cell subpopulations of adult mouse bone marrow, including pluri-potent hematopoietic stem cells. In an effort to understand the function of ER-MP12 antigen in hematopoiesis, we used biochemical and physical methods to determine its identity. ER-MP12 antigen was isolated by immunoprecipitation from FDCP-1 cell membrane proteins, yielding a glycosylated 113-kDa band upon analysis by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Thirteen peptides derived from trypsinized ER-MP12 antigen were analyzed by electrospray ionization mass spectrometry and compared to a protein sequence database. The search revealed the identity of the ER-MP12 antigen as platelet endothelial cell adhesion molecule-1, CD31 (PECAM-1). This result was subsequently confirmed by Edman sequencing of a single ER-MP12 peptide fragment followed by comparison with PECAM-1 sequence. In addition, flow cytometric analysis of bone marrow and embryonic stem cells revealed highly similar profiles between ER-MP12 and CD31 (MEC 13.3 antibody)-stained cells. The presence of PECAM-1 on primitive hematopoietic stem cells supports the theory for the interaction of hematopoietic progenitor and stem cells with bone marrow stroma and transendothelial migration.