The interaction of calmodulin with human erythrocyte spectrin. Inhibition of protein 4.1-stimulated actin binding.

The functional significance of calmodulin binding to human erythrocyte spectrin has been investigated under native conditions. Both native calmodulin and calmodulin derivatized with the photoactivable cross-linker methyl 4-azidobenzimidate (azidocalmodulin) have been used. When azidocalmodulin is photolyzed in the presence of erythrocyte ghosts, ghost extracts, or purified protein, it cross-links predominately to the beta subunit of erythrocyte spectrin. This cross-linking is calcium-dependent, requires photolysis, and is inhibited by 100 microM trifluoperazine or unlabeled calmodulin. Calmodulin labeled spectrin exhibits a specific and non-calcium-dependent inhibition of its ability to bind actin, even in the presence of protein 4.1. Its ability to self-associate or to bind spectrin-depleted membrane vesicles is unperturbed. Native calmodulin also inhibits protein 4.1-stimulated spectrin-actin binding, but unlike that of covalently bound calmodulin, inhibition by the uncross-linked calmodulin requires calcium. The degree of inhibition of spectrin-actin-4.1 binding induced by native calmodulin is significant since 109 microM calmodulin inhibits over 63% of the spectrin-actin binding induced by 4.5 microM protein 4.1. These results demonstrate a specific effect of calmodulin on erythroid spectrin function and suggest that calmodulin may influence the binding of protein 4.1 and actin to spectrin within the cytoskeleton.