Ca2+ Binding to Calmodulin and Its Role in Schizosaccharomyces pombe as Revealed by Mutagenesis and NMR Spectroscopy (∗)

As a first step toward identifying the important structural elements of calmodulin from Schizosaccharomyces pombe, we examined the ability of heterologous calmodulins and Ca2+-binding site mutant S. pombe calmodulins to replace the essential cam1+ gene. A cDNA encoding vertebrate calmodulin allows growth of S. pombe. However, calmodulin from Saccharomyces cerevisiae does not support growth even though the protein is produced at high levels. With one exception, all mutant S. pombe calmodulins with one or more intact Ca2+-binding sites allow growth at 21°C. A mutant containing only an intact Ca2+-binding site 3 fails to support growth, as does S. pombe calmodulin with all four Ca2+-binding sites mutated. Several of the mutant proteins confer a temperature-sensitive phenotype. Analysis of the degree of temperature sensitivity allows the Ca2+-binding sites to be ranked by their ability to support fission yeast proliferation. Site 2 is more important than site 1, which is more important than site 4, which is more important than site 3. A visual colony color screen based on the fission yeast ade1+ gene was developed to perform these genetic analyses. To compare the Ca2+-binding properties of individual sites to their functional importance for viability, Ca2+ binding to calmodulin from S. pombe was studied by 1H NMR spectroscopy. NMR analysis indicates a Ca2+-binding profile that differs from those previously determined for vertebrate and S. cerevisiae calmodulins. Ca2+-binding site 3 has the highest relative affinity for Ca2+, while the affinities of sites 1, 2, and 4 are indistinguishable. A combination of an in vivo functional assay and an in vitro physical assay reveals that the relative affinity of a site for Ca2+ does not predict its functional importance.