Specific conformation and Ca(2+)-binding mode of yeast calmodulin: insight into evolutionary development

The vertebrate calmodulin is configured with two structurally independent globular lobes in N- and C-terminus, and a flexible central linker. Distinctly, two lobes of calmodulin from Saccharomyces cerevisiae (yCaM) interact and influence the Ca(2+)-binding profile of each other. We explored this further using the mutant proteins with eliminated Ca(2+)-binding ability in one of the lobes and found that the Ca(2+)-bound N-lobe associates with the Ca(2+)-free C-lobe to gain the Ca(2+) affinity of a wild-type level. Next, analysing series of C-terminal residue truncation mutant, we found that the truncation of C-terminal three residues induce the hyper Ca(2+) affinity. These residues are also important for the general structural behaviour of calmodulin, such as Ca(2+)-induced slow mobility shift in polyacrylamide gel electrophoresis and for the ability to activate Cmk1p (yeast calmodulin kinase). These suggest: (i) when Ca(2+) occupies only N-lobe, two lobes interact and form the stable intermediate leading to a proper level of Ca(2+) affinity; (ii) the C-terminal three residues are required to prohibit abnormal stabilization of the intermediate promoting abnormally high Ca(2+) affinity and for recognition of target enzymes. A model for Ca(2+) and target bindings of yCaM is proposed. Evolutional aspect concerning the biological significance of this model was discussed.