Intrinsic calcium sensitivity of tubulin polymerization. The contributions of temperature, tubulin concentration, and associated proteins.

The calcium concentration required to inhibit tubulin polymerization by 50% (Ca2+ sensitivity) extends from the micromolar to the millimolar range and is a function of a number of factors that include 1) a steep, inverse dependence on tubulin concentration: two-cycle tubulin has lower Ca2+ sensitivity than pure tubulin (prepared by a novel method described under "Appendix"); 2) temperature: Ca2+ sensitivity shows a steep increase below 24 degrees C; 3) microtubule seeds: these decrease sensitivity to Ca2+ inhibition; 4) the presence of 16 S oligomers or microtubule-associated proteins. Ca2+ increases the critical concentration for microtubule protein and decreases the initial rate of polymerization. All tubulin preparations examined contain small amounts of calmodulin. However, the molar ratio of calmodulin to tubulin is less than 0.01, hence this protein is not required for high Ca2+ sensitivity. Nevertheless, calmodulin at high molar ratios can increase the sensitivity of microtubule assembly toward Ca2+. We conclude that tubulin possesses high intrinsic as well as a calmodulin-mediated Ca2+ sensitivity, and propose that high Ca2+ sensitivity may be a property of the nucleation process.