Control of microtubule assembly-disassembly by calcium-dependent regulator protein.

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Abstract
The [rat testis] Ca2+-dependent regulator (CDR) protein of cyclic nucleotide phosphodiesterase is a low MW, acidic, Ca2+-binding protein which has been implicated in a number of Ca2+-dependent enzymatic functions. Indirect immunofluorescence has revealed that CDR is specifically associated with the chromosome-to-pole region of the mitotic apparatus during metaphase-anaphase [of mouse 3T3 cells] in a pattern distinctly different from that of similar cultured cells stained with anti-tubulin. This characteristic localization in the mitotic half-spindle suggested a role for CDR in the control of microtubule assembly-disassembly during mitosis. CDR was examined for its effects on microtubule polymerization in vitro. Stoichiometric concentrations of CDR and a homologous Ca2+-binding protein, [rabbit] skeletal muscle troponin C, both inhibited and reversed microtubule assembly in a Ca2+-dependent manner. CDR-dependent inhibition of in vitro microtubule assembly occurred at physiological Ca2+ concentrations (.apprx. 10 .mu.M) that, in the absence of CDR, caused only a slight reduction in polymerization. At Ca2+ concentrations in the low physiological range (< 1 .mu.M), no inhibition was observed. These biochemical results, together with the immunofluorescent localization of CDR in the mitotic half-spindle, provide evidence that Ca2+ is an endogenous regulator of microtubule disassembly through the activity of CDR.