Cleavage of connectin by calpain and cathepsin D.

mu-Calpain quickly split the alpha-connectin in myofibrils into beta-connectin, and then produced a 1700-kDa component. Cathepsin D also split alpha-connectin into beta-connectin, further degrading it to fragments smaller than the 1700-kDa component with increasing incubation time. The action of cathepsin D on the connectin molecule was distinctly different from that of mu-calpain in terms of the splitting rate and manner. When freshly excised muscle was exposed to a temperature of 37 degrees C, complete disappearance of connectin (alpha, beta and 1700-kDa component) was observed within 36 h. In contrast, at 2 degrees C, about 75% of connectin was retained as beta-form even after 3 weeks. The present data suggest that the degradation of connectin in muscle might be caused by mu-calpain in the early stage of aging, and then with time, this action is replaced by m-calpain or cathepsin D. However, the possibility of other intrinsic proteases participating in the degradation of connectin still remains.