Mastin is a gelatinolytic mast cell peptidase resembling a mini-proteasome

Mastin is a tryptic peptidase secreted by canine mast cells. This work reveals that mastin is composed of catalytic domain singlets and disulfide-linked dimers. Monomers unite non-covalently to form tryptase-like tetramers, whereas dimers aggregate with monomers into larger clusters stabilized by hydrophobic contacts. Unlike tryptases, mastin resists inactivation by leech-derived tryptase inhibitor, indicating a smaller central cavity, as confirmed by structural models. Nonetheless, mastin is strongly gelatinolytic while not cleaving native collagen or casein, suggesting a preference for denatured proteins threaded into its central cavity. Phylogenetic analysis suggests that mammalian mastins shared more recent ancestors with soluble alpha/beta/delta tryptases than with membrane-anchored gamma-tryptases, and diverged more rapidly. We hypothesize that gelatinase activity and formation of inhibitor-resistant oligomers are ancestral characteristics shared by soluble tryptases and mastins, and that secreted mastin is a mini-proteasome-like complex that breaks down partially degraded proteins without causing bystander damage to intact, native proteins.