Chemical modification of the sole histidine residue of smooth muscle caldesmon

AbstractCaldesmon was stoichiometrically N-carbethoxylated specifically at the only histidine residue (His-610) with diethylpyrocarbonate. Carbethoxylation of a 1:1 molar complex of caldesmon and calmodulin in the presence of Ca2+ resulted in the stoichiometric N-carbethoxylation of His-610 of caldesmon and His-107 of calmodulin. Carbethoxy-caldesmon, like the unmodified protein, bound to immobilized calmodulin (in the presence of Ca2+) and to immobilized tropomyosin (at low ionic strength). The affinity of F-actin for carbethoxy-caldesmon (K4 = 1.29 × 10−4M) was similar to that for unmodified caldesmon (K4 = 0.88 × 10−4M), and the modified protein was as effective as control caldesmon in the inhibition of the actin-activated MgATPase of skeletal muscle myosin. We conclude that the predicted basic amphiphilic α-helical sequence (Arg-593-His-610) does not represent the calmodulin-binding site of caldesmon. Furthermore, His-610 does not play a major role in the interaction of caldesmon with F-actin or tropomyosin