A calcium-protease activator associated with brain microsomal-insoluble elements.

A factor which markedly activates Ca2+-dependent thiol protease (calpain) is associated with Triton X-100-insoluble materials, presumably structural elements such as cytoskeletons, of bovine brain microsomal fraction. This factor is extracted with 0.6 M KC1, and purified partially by sucrose density gradient centrifugation and hydroxyapatite column chromatography. The factor appears to be a heat-stable protein with an approximate Mr of 15 000. With casein as substrate this factor activates both calpain I and calpain II several-fold up to more than 10- fold without alteration of their affinity to Ca2+. Calmodulin is unable to substitute for this factor. A similar factor is associated with human platelet insoluble materials.