Properties and intracellular localization of calpain activator protein

In this paper, we have further analyzed the properties of calpain activator (CA) in order to better define its physiological function. The activator s hows a pH optimum approximately 7.8-8.0, independently of the nature of the buffer used. Although the maximal activity is observed with human acid-den atured globin, the effect of CA is detectable with other protein substrates , such as casein and insulin. A comparable activating effect is observed al so with the synthetic substrate Succ-Leu-Tyr-AMC. The activatory effect has been evaluated in a reconstructed system, using plasma membrane Ca2+-ATPas e as substrate. CA is localized in erythrocyte precursor cells on the inner surface of the plasma membrane in very high amount and its level profoundl y decreases up to 10% of the original value when cells reach the terminal d ifferentiated state. (C) 2000 Academic Press.