When platelet membranes previously treated by 0.6M KI were reacted with 14C-NEM, 9 protein bands including membrane actin were labeled. If KI treated platelet membranes were first reacted with cold NEM, beta-Mercaptoethanol, and 14C-NEM sequentially only three protein bands, one of which was actin, were labeled. These results imply that some of the tightly associated membrane actin thiol groups are free and some of them form disulfide bonds with two other labeled proteins. The candidates that might form disulfide bonds with actin were identified by monoclonal antibody to be GpIIb and/or GpIIIa. Extraction experiments showed that even when the disulfide bonds that link actin to membrane integral protein were first reduced by DTT and then extracted with 0.6M KI, membrane actin still remained tightly associated to the membrane by some other means. Membrane actin could be extracted with 1% octyl glucoside but remained as part of a high-molecular-weight complex. From these results we believe that platelet membrane actin may be partially embedded into the bilayer of the lipid membrane and disulfide linked to membrane integral proteins. It may thus act as a nucleating center for the polymerization of cytosolic actin in the assembly of the cytoskeleton.
Copyright 1998 Academic Press.