Calmodulin requires Ca ²⁺ to integrate into the spindle pole body and modulate the cytokinetic ring constriction in fission yeast cells

Although calmodulin is best known as a calcium sensor, it also possesses Ca ²⁺ -independent functions, exemplified by the surprising finding that the budding yeast cells remain viable using a calmodulin mutant that can’t bind Ca ²⁺ (Geiser et al., 1991). It remains unclear why Ca ²⁺ -calmodulin is required in other yeasts or vertebrates. Here, we determined whether such holo-calmodulin is an essential cytoskeletal protein in fission yeast. The S. pombe calmodulin Cam1 was an integral part of both the spindle pole body (SPB) and many actin structures at the equatorial division plane. Two mutants Cam1-2V and -3V, which bound Ca ²⁺ poorly, reduced their localization at the SPBs by 90%. However, their presence in the actin structures remained unchanged. Replacing the endogenous cam1 with cam1-2V cut the number of the Cam1-interacting protein Pcp1 in the SPB by ∼70% and delayed mitosis. In contrast, the assembly and constriction of the cytokinetic ring, which depends on the Cam1-interacting myosins Myo1, Myo51 and Myo52, accelerated. The temperature-sensitive cam1-2V mutant was rescued by either over-expression of pcp1 or deletion of myo1 . Thus, Cam1 depends on Ca ²⁺ to promote the SPB assembly and to modulate the actomyosin ring constriction, suggesting holo-calmodulin as an essential cytoskeletal protein.