Subcellular distribution and phosphorylation of vinculin isoforms in human blood platelets

In this study we investigated human blood platelet vinculin microheterogeneity, the subcellular localization and phosphorylation of the different isoforms before and after platelet stimulation. At least 5 vinculin isoforms could be detected, as well as meta-vinculin. These isoforms did not demonstrate a specific subcellular localization, i.e. their relative content was similar in cytoskeleton, membrane skeleton and cytosol. Upon platelet stimulation with thrombin a small increase in alpha-vinculin was noted in all platelet subfractions. The cytoskeleton of non-stimulated platelets contained a minor quantity of vinculin. Upon thrombin stimulation of the platelets the cytoskeletal vinculin content increased significantly; previously we already reported a maximal 10% incorporation of the total platelet vinculin content into the cytoskeleton upon stimulation. A phosphorylation of a minor vinculin-isoform, i.e. at the alpha'/alpha location was mainly detected in the cytoskeleton. This phosphorylation was observable in the non-stimulated platelet cytoskeletal vinculin. These findings argue against a regulatory role for vinculin phosphorylation in the uptake of the main isoforms of this protein in the platelet cytoskeleton upon thrombin stimulation. The function of the phosphorylated cytoskeletal vinculin remains to be established.