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We have investigated the functional properties of a mutant (Cg1) derived from the C‐terminal 99 amino acids of chicken caldesmon, 658–756 (658C) where the sequence 691glu‐trp‐leu‐thr‐lys‐thr696is changed to pro‐gly‐his‐tyr‐asn‐asn. Cg1 bound Ca2+‐calmodulin with (1/7)th of the affinity as compared to 658C or whole caldesmon. NMR titrations indicate that the contacts of Ca2+‐calmodulin with the Trp‐722 region of the peptide are retained but that those at the mutated site are lost. Most importantly Ca2+‐calmodulin is not able to reverse the Cg1‐induced inhibition. We conclude that the interaction of calmodulin with this caldesmon sequence is crucial for the reversal of caldesmon inhibition of actin‐tropomyosin activation of myosin ATPase. The results are interpreted in terms of multi‐site attachment of actin and Ca2+‐calmodulin to overlapping sequences in caldesmon domain 4b.

We have investigated the functional properties of a mutant (Cg1) derived from the C-terminal 99 amino acids of chicken caldesmon, 658–756 (658C) where the sequence 691glu-trp-leu-thr-lys-thr 696is changed to pro-gly-his-tyr-asn-asn. Cg1 bound Ca 2+-calmodulin with (1/7)th of the affinity as compared to 658C or whole caldesmon. NMR titrations indicate that the contacts of Ca 2+-calmodulin with the Trp-722 region of the peptide are retained but that those at the mutated site are lost. Most importantly Ca 2+-calmodulin is not able to reverse the Cg1-induced inhibition. We conclude that the interaction of calmodulin with this caldesmon sequence is crucial for the reversal of caldesmon inhibition of actin-tropomyosin activation of myosin ATPase. The results are interpreted in terms of multi-site attachment of actin and Ca 2+-calmodulin to overlapping sequences in caldesmon domain 4b.