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Abstract A protein fraction that confers 1,2-bis-(2-dicarboxymethylaminoethoxy)ethane (EGTA) sensitivity on reconstituted actomyosin made with highly purified actin and myosin has been prepared from actin extracted from the acetone-dried muscle powder at 25–37°. This protein fraction had effects upon reconstituted actomyosin that, in the absence of Ca++-binding agents, resembled those of tropomyosin made by classical methods in that the Mg++-activated adenosine triphosphatase activity was inhibited during the clearing phase and enhanced after superprecipitation. Unlike tropomyosin made by classical methods, this protein fraction potentiated an inhibitory action of EGTA upon the Mg++-activated ATPase activity of reconstituted actomyosin. In the presence of Mg++ and the protein fraction, EGTA appeared to prevent activation of myosin ATPase by actin. The EGTA-sensitizing protein fraction, which sedimented as a single symmetrical peak in the ultracentrifuge, had a sedimentation coefficient of 2.66 S, a value similar to that of tropomyosin. The dependence of specific viscosity upon ionic strength was similar for both. However, the intrinsic viscosity of the protein fraction was less than that of tropomyosin, and amino acid analyses revealed several significant differences between the two. The relatively high contents of proline and phenylalanine found in the EGTA-sensitizing protein fraction indicate that a previously undescribed protein was present in addition to tropomyosin. It is possible that this additional protein material was responsible for sensitization of actomyosin to Ca++-binding agents, but the possibility remains that this action was produced by a native form of tropomyosin.
Arnold M. Katz (Fri,) studied this question.