Type II metacaspases Atmc4 and Atmc9 of Arabidopsis thaliana cleave substrates after arginine and lysine

Nine potential caspase counterparts, designated metacaspases, were identified in the Arabidopsis thaliana genome. Sequence analysis revealed two types of metacaspases, one with ( type I) and one without ( type II) a proline- or glutamine-rich N-terminal extension, possibly representing a prodomain. Production of recombinant Arabidopsis type II metacaspases in Escherichia coli resulted in cysteine-dependent autocatalytic processing of the proform into large and small subunits, in analogy to animal caspases. A detailed biochemical characterization with a broad range of synthetic oligopeptides and several protease inhibitors of purified recombinant proteins of both metacaspase 4 and 9 showed that both metacaspases are arginine/lysine-specific cysteine proteases and did not cleave caspase-specific synthetic substrates. These findings suggest that type II metacaspases are not directly responsible for earlier reported caspase-like activities in plants.