[Profilament actin in muscle cells of starfish ambulacra]

In gelatinated extracts of starfish ambulacral podia the actin polymerized during gel formation exists as oligomers incapable of binding to rabbit myosin. Actin is also the major protein extractable from the cells by glycerol- and Triton-containing solutions, which do not destroy the contractile mechanism of the cells. The weakly bound gel proteins washed from gel by low ionic strength solutions comprise a factor hampering the polymerization of rabbit actin. The data obtained suggest that the ambulacral sarcoplasmic proteins contain actin (profilament form) coupled to the factor preventing its polymerization.