Calmodulin and the kinesin-like calmodulin-binding protein in plant cell division

The roles of the calcium binding messenger protein calmodulin and the kinesin-like calmodulin binding protein during cell division in plants were investigated employing microinjection of various probes in Tradescantia virginiana stamen hair cells. Fluorescently labeled calmodulin was found to distribute uniformly throughout the cytoplasm, and was not specifically colocalized with the microtubules of the dividing cell as has been reported earlier. Injected calmodulin redistributed during fixation as if for immunolocalization, suggesting that earlier reports were based on artifactual localizations, and consequently that the role of calmodulin in cell division should be reevaluated. Inhibition of calmodulin using a synthetic calmodulin binding peptide, prevented the actomyosin based cytoplasmic streaming and the late lateral expansion phase of cytokinesis, but did not affect the architecture of cytoplasmic strands or anaphase chromosome motion. Thus, although calmodulin is not localized specifically to the phragmoplast, it does play a regulatory role during cytokinesis. Microinjection of antibodies to the kinesin-like calmodulin binding protein (KCBP) that constitutively activate this microtubule motor protein, caused precocious nuclear envelope breakdown, metaphase arrest, and delays in cytokinesis. However, the antibodies did not inhibit chromosome motion. These results, in combination with earlier characterization of this kinesin, lead to the conclusion that KCBP is differentially involved in forming a converging bipolar spindle. Interfering with its regulation affects the dynamic transformations of the microtubule cytoskeleton between these phases. The results establish a regulatory role for calmodulin in cell division and address two possible downstream signaling pathways: inhibition of myosins involved in the lateral expansion of the phragmoplast, and inhibition of the spindle forming kinesin-like calmodulin binding protein