Analysis of the molecular basis of calmodulin defects that affect ion channel-mediated cellular responses: site-specific mutagenesis and microinjection

The ability of microinjected calmodulin to temporarily restore an ion channel-mediated behavioral phenotype of a calmodulin mutant in Paramecium tetraurelia (cam1) is dependent on the amino acid side chain that is present at residue 101, even when there is extensive variation in the rest of the amino acid sequence. Analysis of conservation of serine-101 in calmodulin suggests that the ability of calmodulin to regulate this ion channel-associated cell function may be a biological role of calmodulin that is widely distributed phylogenetically. A series of mutant calmodulins that differ only at residue-101 were produced by in vitro site-specific mutagenesis and expression in Escherichia coli, purified to chemical homogeneity, and tested for their ability to temporarily restore a wild-type behavioral phenotype to cam1 (pantophobiacA1) Paramecium. Calmodulins with glycine-101 or tyrosine-101 had minimal activity; calmodulins with phenylalanine-101 or alanine-101 had no detectable activity. However, as a standard of comparison, all of the calmodulins were able to activate a calmodulin-regulated enzyme, myosin light chain kinase, that is sensitive to point mutations elsewhere in the calmodulin molecule. Overall, these results support the hypothesis that the structural features of calmodulin required for the transduction of calcium signals varies with the particular pathway that is being regulated and provide insight into why inherited mutations of calmodulin at residue 101 are nonlethal and selective in their phenotypic effects.