Nonessential role for methionines in the productive association between calmodulin and the plasma membrane Ca-ATPase

To investigate the role of hydrophobic interactions involving methionine side chains in facilitating the productive association between calmodulin (CaM) and the plasma membrane (PM) Ca-ATPase, we have substituted the polar amino acid Gln for Met at multiple positions in both the amino- and carboxyl-terminal domains of CaM. Conformationally sensitive fluorescence signals indicate that these mutations have little effect on the backbone fold of the carboxyl-terminal domain of CaM. The insertion of multiple Gln in either globular domain results in a decrease in the apparent affinity of CaM for the PM-Ca-ATPase. However, despite the multiple substitution of Gln for four methionines at positions 36, 51, 71, and 72 in the amino-terminal domain or for three methionines at positions 124, 144, and 145 in the carboxyl-terminal domain, these mutant CaMs are able to fully activate the PM-Ca-ATPase. Thus, although these CaM mutants have a decreased affinity for the CaM-binding site on the Ca-ATPase, they retain the ability to fully activate the Ca-ATPase at saturating concentrations of CaM. The role of individual methionines in modulating the affinity between the carboxyl terminus and the PM-Ca-ATPase was further investigated through the substitution of individual Met with Gln. Upon substitution of Met(124) and Met(144) with Gln, there is a 5- and 10-fold increase in the amount of CaM necessary to obtain half-maximal activation of the PM-Ca-ATPase, indicating that these methionine side chains participate in the high-affinity association between CaM and the PM-Ca-ATPase. However, substitution of Gln for Met(145) results in no change in the apparent affinity between CaM and the PM-Ca-ATPase, indicating that in contrast to all other known CaM targets, Met(145) does not participate in the interaction between CaM and the PM-Ca-ATPase. These results emphasize differences in the binding interactions between individual methionines in CaM and different target enzymes, and suggest that hydrophobic interactions between methionines in CaM and the binding site on the PM-Ca-ATPase are not necessary for enzyme activation. Calculation of the binding affinities of individual CaM domains associated with activation of the PM-Ca-ATPase suggests that mutations of methionines located in either domain of CaM can decrease the initial high-affinity association between CaM and the PM-Ca-ATPase, but have little effect upon the subsequent binding of the opposing globular domain. These results suggest that the initial associations between CaM and the CaM-binding sequence in the PM-Ca-ATPase are guided by nonspecific hydrophobic interactions involving both domains of CaM.