The structural, functional, and therapeutic potential of metacaspases in fungi and protozoa.

Metacaspases are cysteine proteases found in fungi, protozoa, and plants, where they regulate critical cellular processes such as programmed cell death (PCD), cell cycle progression, and protein homeostasis. Although structurally related to caspases, metacaspases differ in mechanism of activation, substrate specificity, and biological roles. Unlike caspases, metacaspases are monomeric calcium-dependent enzymes that cleave substrates after basic residues such as arginine or lysine. This review provides a comprehensive overview of the structural classification, biochemical regulation, and physiological functions of metacaspases in model eukaryotes. We discuss their roles in stress adaptation, cell death, and proteostasis in organisms such as Saccharomyces cerevisiae, Candida albicans, Trypanosoma brucei, and Trypanosoma cruzi. We highlight recent advances in understanding their activation via calcium binding and autocatalytic processing, and explore their functional diversity across species. In addition, we examine the therapeutic potential of metacaspases as drug targets due to their absence in mammals and essential roles in pathogenic microbes. Challenges in substrate identification, enzymatic characterization, and inhibitor design are also addressed, along with emerging tools that may accelerate metacaspase research. Altogether, this review underscores the growing importance of metacaspases in eukaryotic biology and their promising applications in antifungal and antiparasitic drug development.