Dimerization of P-selectin in platelets and endothelial cells

P-selectin is a leukocyte adhesion receptor stored in platelets and endothe lial cells and is translocated to the surface upon cell activation. Purifie d P-selectin is oligomeric and has increased avidity for its ligand relativ e to the monomeric form, but whether P-selectin self-associates in the memb rane of intact cells is not known. A chemical cross-linking approach was us ed to show that P-selectin is present as noncovalent dimers in resting plat elets, human umbilical vein endothelial cells, and heterologous RIN5F cells expressing P-selectin, The results of 5-dimensional isoelectric focusing a re consistent in showing P-selectin dimers as homodimers, but they are comp osed of a more basic subset of P-selectin than the monomers. This suggests that the dimers are a biochemically distinct subset of P-selectin, P-select in dimers form in the endoplasmic reticulum and Golgi compartments of human umbilical vein endothelial cells only after synthesis of the mature P-sele ctin subunit, and are not preferentially stored in Weibel-Palade bodies as compared with the monomeric form. Platelet activation with thrombin recepto r-activating peptide leads to the presence of P-selectin monomers and homod imers on the cell surface as well as P-selectin heterodimers, which are com -posed of P-selectin and an unidentified protein of approximately 81 kd mol ecular weight. In summary, these studies demonstrate that P-selectin Is hom odimeric in situ and that platelet activation leads to the formation of an additional activation-specific heterodimeric species. In addition, the homo dimer has unique biochemical characteristics compared with the monomeric fo rm, and dimerization occurs in the endoplasmic reticulum and Golgi compartm ents of endothelial cells. (C) 2000 by The American Society of Hematology.