Characterization of a calcium-activated protease that hydrolyzes a microtubule-associated protein

Rat brain extract contains a calcium-activated protease that hydrolyzes the high-molecular-weight microtubule-associated protein MAP₂. We have purified this enzyme greater than 30-fold by pH precipitation, DEAE, and Bio-Gel P-300 chromatography. The partially purified enzyme is half-maximally activated by 2 μm calcium when assayed using 2× cycled microtubules as substrate. The enzyme is active over a pH range of 6.5 to 7.5 and is heat labile and temperature dependent. The calcium-dependent regulator, calmodulin, cannot replace the protease in promoting MAP₂ hydrolysis. The partial purification of a calcium-activated protease from brain tissue explains the sensitivity of in vitro tubulin polymerization to micromolar concentrations of calcium.