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 predominantly associated
with the membrane of platelet α-granules and it is translocated to the plasma membrane
upon platelet activation (Metzelaar et al. Blood 1992; 79: 372-9). In this study we
have isolated the protein by immunoaffinity chromatography. The N-terminus was sequenced
and was identical to the N-terminal sequence of human thrombospondin. The protein
was N-glycosylated and bound to heparin as would be expected of the N-terminal part
of thrombospondin. RUU-SP 1.77 reacted only with reduced thrombospondin. Plasmin and
trypsin digestion of thrombospondin yielded fragments of approximately 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 associated.
Keywords
Thrombospondin - platelet α-granule - platelet activation - platelet membrane