The possible self-down-regulation of calpain triggered by cell membranes

摘要

In order to confirm whether the binding sites for μ-calpain on the inner surface of erythrocyte membranes are substrate proteins themselves, we examined the binding properties of μ-calpain to μ-calpain-pretreated inside-out membranes. When native μ-calpain was incubated with μ-calpain-pretreated membranes, however, newly added calpain was degraded rapidly in a time- and Ca2+-dependent manner. Although the degradation of la,-calpain was not inhibited by various proteinase inhibitors, it was strongly inhibited by digestible substrates for calpain that possess the ability to inhibit the binding of μ-calpain to erythrocyte membranes. On the other hand, when μ-calpain inactivated by N-ethyllmaleimide was incubated with μ-calpain-pretreated membranes, no degradation was observed. These results indicate that the degradation of μ-calpain occurs on the surface of μ-calpain-modified membranes and that it depends on the autoproteolytic activity of μ-calpain itself. It seems likely that the autoproteolytic activity of μ-calpain is accelerated markedly by some component(s) exposed on the surface of membranes during the pretreatment with μ-calpain. The possibility is thus proposed that cell membranes possess the ability to down-regulate calpain to protect cell membranes from overdegradation by excessively bound calpain. The active factor(s) in the membranes that can accelerate the autoproteolytic degradation of lt,-calpain could be almost completely removed from μ-calpain-modified membranes by treatment with Triton X-100.