Cytoplasmic tubulin from the unfertilized sea urchin egg: II. Variation of the intrinsic calcium sensitivity of Strongylocentrotus purpuratus egg tubulin as a function of temperature and brain microtubule-associated proteins.

Cytoplasmic tubulin purified from unfertilized sea urchin eggs self-assembles in the absence of microtubule-associated proteins (MAPs) [Suprenant and Rebhun, 1983; Detrich and Wilson, 1983] with a critical concentration for polymerization of 0.8 mg/ml at 15-18 degrees C, a value well below the 3 mg/ml tubulin present in these eggs [Pfeffer et al, 1976]. Studies of the calcium sensitivity of unfertilized S. purpuratus (sea urchin) egg tubulin were initiated to help understand how this tubulin is maintained unassembled in the unfertilized egg. Egg microtubules, assembled at physiological temperatures (15-18 degrees C) were depolymerized by a 100-fold lower free calcium concentration than egg microtubules assembled at the higher temperatures (25-37 degrees C) generally used to assemble mammalian brain microtubules. The initial rate of egg microtubule assembly was much more sensitive to calcium than was microtubule depolymerization at steady state at 37 degrees C. However, both processes were sensitive to near physiological free calcium concentrations at 18 degrees C. The co-assembly of bovine brain MAPs and sea urchin egg tubulin produced microtubules that required a 1,000-fold higher concentration of free calcium for depolymerization than microtubules assembled at 18 degrees C from egg tubulin alone. While calcium regulatory MAPs have not yet been found in sea urchin eggs, the fact that brain MAPs interact with egg tubulin and regulate both its critical concentration for polymerization [Suprenant and Rebhun, 1983] and its calcium sensitivity, suggests that such regulatory molecules exist. These results suggest that sea urchin egg tubulin assembly in vivo could be controlled by variations in intracellular calcium levels acting in concert with urchin egg proteins similar in function to brain MAPs.