The 33-kDa platelet alpha-granule membrane protein (GMP-33) is an N-terminal proteolytic fragment of thrombospondin

GMP-33 is a platelet membrane associated protein that is recognised by RUU- SP 1.77, an antibody raised against activated platelets. GMP-33 is predomin antly associated with the membrane of platelet alpha -granules and it is tr anslocated to the plasma membrane upon platelet activation (Metzelaar et al . Blood 1992; 79: 372-9). In this study we have isolated the protein by imm unoaffinity chromatography. The N-terminus was sequenced and was identical to the N-terminal sequence of human thrombospondin, The protein was N-glyco sylated and bound to heparin as would be expected of the N-terminal part of thrombospondin. RUU-SP 1.77 reacted only with reduced thrombospondin. Plas min and trypsin digestion of thrombospondin yielded fragments of approximat ely the same size as GMP 33 that reacted with RUU-SP 1.77 after reduction. No evidence for alternative splicing was found. We postulate that GMP 33 is an N-terminal proteolytic fragment of thrombospondin that is membrane asso ciated.