Serine/threonine-protein kinase which extends lifespan and delays tissue aging, probably by activating daf-16. ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Interacts with rsf-1 (via SARAH domain); the interaction is required for the phosphorylation of cst-1. Widely expressed in epidermal cells. Proteolytically cleaved by caspase-3 during apoptosis which results in kinase activation. Phosphorylated. Belongs to the protein kinase superfamily. STE Ser/Thr protein kinase family. STE20 subfamily.