Strongylocentrotus purpuratus spindle tubulin. II. Characteristics of its sensitivity to Ca++ and the effects of calmodulin isolated from bovine brain and S. purpuratus eggs.

Tubulin was extracted from spindles isolated from embryos of the sea urchin Strongylocentrotus purpuratus and purified through cycles of temperature-dependent assembly and disassembly. At 37 degrees C, the majority of the cycle-purified spindle tubulin polymer is insensitive to free Ca++ at concentrations below 0.4 mM, requiring free Ca++ concentrations greater than 1 mM for complete depolymerization. However, free Ca++ at concentrations above 1 microM inhibits initiation of polymer formation without significantly inhibiting the rate of elongation onto existing polymer. At 15 degrees C and 18 degrees C, temperatures that are physiological for S. purpuratus embryos, spindle tubulin polymer is sensitive to free Ca++ at micromolar concentrations such that 3-20 microM free Ca++ causes complete depolymerization. Calmodulin purified from either bovine brain or S. purpuratus eggs does not affect the Ca++ sensitivity of the spindle tubulin at 37 degrees C, although both increase the Ca++ sensitivity of cycle-purified bovine brain tubulin. These results indicate that cycle-purified spindle tubulin and cycle-purified bovine brain tubulin differ significantly in their responses to calmodulin and in their Ca++ sensitivities at their physiological temperatures. They also suggest that, in vivo, spindle tubulin may be regulated by physiological levels of intracellular Ca++ in the absence of Ca++-sensitizing factors.