Association ofRalGTP-Binding Protein with Human Platelet Dense Granules

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GTP-binding proteins of molecular mass of 24–27 kDa were detected in the dense granule fraction of human platelets when nitrocellulose blots containing proteins separated by SDS-polyacrylamide gel electrophoresis were incubated with [α-32P]GTP. Further analysis, using isoelectric focusing and two-dimensional polyacrylamide gel electrophoresis, resolved the dense granule 27 kDa and 24 kDa GTP-binding proteins into four distinct forms each. GTP-binding proteins in the total particulate fraction were resolved into seven 27 kDa and four 24 kDa forms. Immunoblotting with antiserum against known platelet low molecular mass GTP-binding proteins demonstrated thatrap2 andG25K/CDC42Hs proteins, although present in platelets, were not detected in the dense granule fraction. However,ralwas one of the proteins associated with dense granules. Association of specific low molecular mass GTP-binding proteins with dense granules suggests a potential role for these proteins in regulating the release of storage contents from this granule.