Important similarities are reported between human smooth muscle actomyosin and the human erythrocyte spectrin complex, primarily components 1,2, and 5 (Fairbanks, G., Steck, T. L., and Wallach, D. F. H. (1971), Biochemistry I0, 2606). The actin-like protein, component 5, is identical with human uterine actin in its ability to form 50-70-Å filaments, to stimulate myosin ATPase activity, and to bind rabbit heavy meromyosin specifically. Antibodies to human smooth muscle myosin(uterine) were prepared which were monospecific. A weak but specific cross-reaction of these antisera with components 1 and/or 2 (spectrin) was characterized and at least 25% of the antimyosin antibodies showed a low affinity reaction with spectrin. Antibodies generated against a soluble complex of spectrin components 1 and 2 reacted only with component 1 and did not cross-react with myosin. In addition to these structural similarities between smooth muscle actomyosin and the spectrin complex, we have found that spectrin is involved in ATP-dependent erythrocyte shape changes (Sheetz, M. P., Painter, R. G., and Singer, S. J. (1976b), Cold Spring Harbor Symp. Cell Motility (in press) and, therefore, the spectrin complex is also a mechanochemical protein system.
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