Mediates digestion of the cell walls of the 2 mating type gametes during mating as a necessary prelude to cell fusion. This enzyme acts specifically on the framework proteins (inner wall) of the cell wall, cleaving several model peptides at specific sites. Cleavage of the proline- and hydroxyproline-rich proteins of the Chlamydomonas cell wall. Also cleaves azocasein, gelatin and Leu-Trp-Met-|-Arg-Phe-Ala. Binds 1 zinc ion per subunit. Stored in the periplasm of gametes until its release. Secreted concurrently with release of the cell walls. By the signal of flagellar agglutination between gametes of the opposite mating type. The conserved cysteine present in the cysteine-switch motif binds the catalytic zinc ion, thus inhibiting the enzyme. The dissociation of the cysteine from the zinc ion upon the activation-peptide release activates the enzyme. Present in 2 forms: an inactive V-form in vegetative cells and an active and soluble G-form. The V-form enzyme may be converted to the G-form enzyme during gametic differentiation under nitrogen-starved conditions. Belongs to the peptidase M11 family.