α -Actinin and Membrane Glycoprotein IIIa are Different Proteins in Human Blood Platelets

It has been suggested that a platelet protein that is very similar to muscle α -actinin is identical to the membrane glycoprotein IIIa (GPIIIa) of platelets and is responsible for anchoring actin filaments directly into the plasma membrane of platelets. To determine if α -actinin and GPIIIa are related in platelets, we analyzed the purified proteins on 5% sodium dodecyl sulfate/polyacrylamide gels. The two proteins differ in mobility in both the unreduced and reduced states, and they stain differently with silver stain. In addition, α -actinin is a prominent component of the detergent-insoluble cytoskeletons of platelets, whereas GPIIIa is absent from these structures. By using monospecific antisera to the individual proteins, it was demonstrated that α -actinin and GPIIIa are immunologically distinct. We conclude that α -actinin and GPIIIa are different proteins in human blood platelets and that it is unlikely that α -actinin is an integral membrane protein.