Similarities and dissimilarities between calmodulin and a Chlamydomonas flagellar protein

A protein that resembles vertebrate calmodulins and troponin C has been isolated from Chlamydomonas flagella by using a calmodulin purification protocol that included calcium-dependent affinity-based adsorption chromatography on phenothiazine-Sepharose conjugates. The flagellar protein resembled calmodulin in elution from reverse-phase columns, had a peptide map similar to that of calmodulin, and competed with vertebrate calmodulin in a radioimmunoassay using antisera against vertebrate calmodulin. However, this flagellar protein did not activate phosphodiesterase, lacked N epsilon-trimethyllysine, and had an isoelectric point approximately 0.3 pH unit higher than that of vertebrate calmodulin. When analyzed by polyacrylamide gel electrophoresis under various conditions, the Chlamydomonas protein migrated between vertebrate calmodulins and rabbit skeletal muscle troponin C and did not manifest a large calcium-dependent mobility shift. This calmodulin-like protein was identified as one of the approximately 200 35S-labeled components in Chlamydomonas flagella resolved by two-dimensional gel electrophoresis. These studies indicate that calmodulin and a structurally and functionally homologous protein are present in the same cell. These studies also demonstrate that caution is necessary: (i) in identifying a protein as a calmodulin, (ii) in using phenothiazines or antisera directed against vertebrate calmodulins as specific probes for calmodulin, and (iii) in the interpretation of experiments on biological systems in which calmodulin is substituted for the homologous calmodulin-like protein.