Isolation and characterization of a calpain activator in chicken skeletal muscle

The calpains (E.C. 3.4.22.17) and calpastatin constitute an ubiquitous, intracellular, Ca2+-dependent protease/inhibitor system. This system has been implicated as a principal regulator of myofibrillar protein degradation in both ante-mortem and postmortem muscle. Although proteolytic activity of the calpains is primarily controlled through interaction of calpain and calpastatin, evidence for an activator(s) has been limited and the reported characteristics varied. The function of the activator has not been elucidated. A putative calpain activator has been isolated from the Pectoralis muscle of broiler breeders (Cobb x Cobb). The activator elutes from an ion-exchange column at approximately 200 mM NaCl. Addition of activator increased apparent m-calpain activity to a level demonstrating a fourfold increase in proteolysis. The activator/calpain complex maintains a requirement for Ca2+ for proteolytic activity. Under physiological conditions, presence of the activator negates the ability of calpastatin to inhibit m-calpain. Additionally, the activator alone does not demonstrate proteolytic activity. Effect of the activator is pH-dependent; in a physiological pH range, the activator enhances m-calpain proteolytic activity but at pH less than 6.75 the effect is to inhibit m-calpain. The activator's ability to modulate m-calpain activity and eliminate calpastatin's effect provides a further means of regulating this important enzyme system.