Metacaspases are ancestral homologs of caspases that can either promote cell death or confer cytoprotec-tion. Furthermore, yeast (Saccharomyces cerevisiae) metacaspase Mca1 possesses dual biochemical activ-ity: proteolytic activity causing cell death and cytoprotective, co-chaperone-like activity retarding replicative aging. The molecular mechanism favoring one activity of Mca1 over another remains elusive. Here, we show that this mechanism involves calmodulin binding to the N-terminal pro-domain of Mca1, which prevents its proteolytic activation and promotes co-chaperone-like activity, thus switching from pro-cell death to anti -ag-ing function. The longevity-promoting effect of Mca1 requires the Hsp40 co-chaperone Sis1, which is neces-sary for Mca1 recruitment to protein aggregates and their clearance. In contrast, proteolytically active Mca1 cleaves Sis1 both in vitro and in vivo, further clarifying molecular mechanism behind a dual role of Mca1 as a cell-death protease versus gerontogene
