Specific regulation of protein phosphatase 2A PR72/B′′ subunits by calpain

Abstract

Protein phosphatase 2A (PP2A) represents a family of multimeric serine/threonine phosphatases with pleiotropic roles in signal transduction. We previously described the functional analysis of two Ca2+-binding EF-hands in the PR72/B′′ class of regulatory PP2A subunits. Now we report partial degradation of PR72/B”α2 and PR130/B”α1 into a 45–48kDa proteolysis-resistant fragment (‘PR45’) by the Ca2+-dependent protease m-calpain. This limited proteolysis is dependent on EF-hand integrity, independent of two PEST-domains, and highly specific as PP2AC, PR65/A and representatives of PR55/B and PR61/B’ subunit families are calpain-resistant. PR45 was also generated in staurosporine-induced apoptotic MCF7 cells in a calpain-dependent way. Calpain treatment weakens the PR72–core enzyme interaction, activates basal PP2AT72 phosphatase activity and dramatically increases its sensitivity for and activation by polycations. This unique property can be exploited in a specific biochemical assay for these holoenzymes. We propose local calpain action in vivo may constitute a novel regulatory mechanism of these holoenzymes.