Both phosphorylation and caspase-mediated cleavage contribute to regulation of the Ste20-like protein kinase Mst1 during CD95/Fas-induced apoptosis

The serine/threonine kinase Mst1, a mammalian homolog of the budding yeast Ste20 kinase, is cleaved by caspase-mediated proteolysis in response to apo ptotic stimuli such as ligation of CD95/Fas or treatment with staurosporine . Furthermore, overexpression of Mst1 induces morphological changes charact eristic of apoptosis in human B lymphoma cells. Mst1 may therefore represen t an important target for caspases during cell death which serves to amplif y the apoptotic response. Here we report that Mst1 has two caspase cleavage sites, and we present evidence indicating that cleavage may occur in an or dered fashion and be mediated by distinct caspases, We also show that caspa se-mediated cleavage alone is insufficient to activate Mst1, suggesting tha t full activation of Mst1 during apoptosis requires both phosphorylation an d proteolysis, Another role of phosphorylation may be to influence the susc eptibility of Mst1 to proteolysis. Autophosphorylation of Mst1 on a serine residue close to one of the caspase sites inhibited caspase-mediated cleava ge in vitro, Finally, Mst1 appears to function upstream of the protein kina se MEKK1 in the SAPK pathway. In conclusion, Mst1 activity is regulated by both phosphorylation and proteolysis, suggesting that protein kinase and ca spase pathways work in concert to regulate cell death.