Platelet factor 4 (PF4) is a 7.8-kD protein synthesized by megakaryocytes, stored in a-granules as a noncovalent tetramer and released from activated platelets. Each monomer has a conformational flexible N-terminal region that is anchored by two disulfide bridges to the protein core, which consists of three antiparallel P -strands and a carboxyl-terminal cc-helix. The mechanism of action of PF4 remains poorly understood, because no cell surface receptor has been yet identified. The basic nature of the C-ter...