Localisation of pp60c-src to the surface membrane of human platelets

In platelets, pp60c-src has been variously localised to cytosol, dense bodies, or plasma membrane and surface-connected canalicular membranes. We have investigated the localisation of pp60c-src in highly purified human platelet subcellular fractions isolated using density gradient centrifugation and continuous flow electrophoresis followed by quantitative immunoblotting. We have obtained fractions enriched in surface membranes and alpha granule membranes as assessed by the presence of the marker proteins gpIb, gpIIb, thrombospondin, fibrinogen and fibronectin. Quantitative immunoblotting demonstrated that most of the cellular pp60c-src co-purified with markers for surface membrane components and that unlike the situation in fibroblasts, platelet pp60c-src did not associate with non-granule intracellular membranes such as Golgi, endoplasmic reticulum and endosomes. A small subpopulation of the total cellular pp60c-src co-purified with granule components or dense body markers, however this occurred in proportion to the presence of surface membrane markers. Our results demonstrate that the majority of platelet pp60c-src is located on the cytoplasmic face of the plasma membrane in human platelets.