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Human blood platelets, which are highly motile cells essential for the maintenance of hemostasis, contain large quantities of actin and other contractile proteins . We have previously introduced a method (Lucas, R. C., T. C. Detwiler, and A . Stracher, J. Cell Biol ., 1976, 70(2, Pt . 2) :259 a) for the quantitative recovery of the platelets' cytoskeleton using a solution containing 1% Triton X-100 and 10 mM EGTA . This cytoskeleton contains most of the platelets' actin, actin-binding protein (ABP, subunit molecular weight = 260,000), and a 105,000-dalton protein. Negative staining of this Triton-insoluble residue on an EM grid shows it to consist of branched cables of actin filaments aligned in parallel .When this cytoskeletal structure is dissolved in high-salt solutions, the actin and ABP dissociate and can subsequently be separated . Here we will present simple and rapid methods for the individual purifications of platelet actin and platelet ABP.When purified actin and ABP are recombined in vitro, they are shown to be both necessary and sufficient for the reformation of the cytoskeletal complex. The reformed structure is visualized as a complex array of fibers, which at the EM level are seen to be bundles of actin filaments. The reformation of the cytoskeleton requires only that the actin be in the filamentous form-no accessory proteins, chelating agents, divalent cations, or energy sources are necessary.In vivo, however, the state of assembly of the platelets' cytoskeleton appears to be under the control of the intracellular concentration of free calcium . Under conditions where proteolysis is inhibited and EGTA is omitted from the Triton -solubiIization step, no cytoskeleton can be isolated . The ability of Ca 12 to control the assembly and disassembly of the platelets' cytoskeleton provides a mechanism for cytoskeletal involvement in shape change and pseudopod formation during platelet activation .

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Chicken myosin heavy chains from adult fast white muscle fibers (both normal and dystrophic), adult slow red fibers, and embryonic presumptive fast white fibers were compared by sodium dodecyl sulfate/polyacrylamide gel electrophoresis and by peptide mapping. The heavy chain of slow red myosin migrated electrophoretically more slowly than the heavy chains of the other myosins and differed markedly from them in its peptide maps. The heavy chain of dystrophic fast white myosin was similar to its normal counterpart by peptide mapping but showed slight differences. The peptide map of the heavy chain of embryonic presumptive fast white myosin had the general features of that of the heavy chain of fast white, not slow red, fibers but contained definite differences from the former. The results are consistent with the existence of a separate gene for the heavy chain of embryonic presumptive fast white myosin.

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