Calpain-mediated degradation of rapidly and slowly axonally transported proteins in retinal ganglion cells.

Labelled axonally transported proteins belonging to four different phases of transport in the retinal ganglion cells of the rabbit were used as substrates in order to study proteolytic degradation in axons and nerve terminals. Proteins of both rapidly and slowly transported phases of axonal transport were easily degraded in small intact pieces of the superior colliculus. Addition of the Ca-dependent neutral protease, calpain, to isolated soluble and membrane fractions from the superior colliculus resulted in an increased rate of degradation of axonally transported components. The effects of calpain was most marked toward components in phases II and V of axonal transport in this system (Karlsson and Sjöstrand, 1971; Willard and Hulebak, 1977). The latter phase contains slowly transported neurofilament and microtubular protein while the former one contains rapidly transported membrane proteins.