alpha-Actinin deficiency in thrombasthenia: possible identity of alpha-actinin and glycoprotein III

Blood platelets contain a variety of contractile protein species, including the glycoprotein alpha-actinin, which is found at the Z disc in skeletal muscle cells. In the present study, we have considered the possibility that alpha-actinin might be one of several previously described platelet surface glycoproteins. Purified anti-alpha-actinin antibody was found to react strongly with partially purified platelet glycoprotein III, weakly with platelet glycoprotein IIb, and not at all with platelet glycoproteins Ib and IV. Platelets from three siblings with thrombasthenia, a disorder characterized by severe deficiency of platelet glycoproteins IIb and III, were found also to be equally deficient in alpha-actinin. These findings indicate that alpha-actinin and glycoprotein III are identical and suggest that this protein may be an anchor point for actin on the inside of the membrane. Combined with ultrastructural studies of normal and thrombasthenic platelets, the new findings provide a clearer understanding of contraction in single cells and small aggregates.