Relations of the Spectrin Complex of Human Erythrocyte Membranes to the Actomyosins of Muscle Cell

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.