Unconventional Activation Mechanisms of MMP-26, a Human Matrix Metalloproteinase with a Unique PHCG XXD Cysteine-switch Motif

ProMMP-26 has the unique Pro-His81-Cys-Gly-Xaa-Xaa-Asp cysteine-switch motif that discriminates this protease from all other matrix metalloproteinases (MMPs) known so far. The conserved, free cysteine residue of the conventional PRC XXPD sequence interacts with the zinc ion of the catalytic domain and provides the fourth coordination site for the catalytic zinc, thereby preventing latent proMMPs from becoming active. MMPs become functionally active when proteolytic cleavage releases the prodomain and the PRC XXPD sequence and exposes the zinc atom. Here, we report that the Pro-His81-Cys-Gly-Xaa-Xaa-Asp motif is not functional in proMMP-26 and consequently is not involved in the activation mechanisms. Organomercurial treatment failed to activate proMMP-26. The autolytic Lys-Lys-Gln59↓Gln60-Phe-His cleavage upstream of the Pro-His81-Cys-Gly-Xaa-Xaa-Asp motif induced the proteolytic activity of recombinant proMMP-26 whereas any further cleavage inactivated the enzyme. The His81 → Arg81 mutation restored the conventional cysteine-switch sequence in the prodomain but failed to induce the cysteine-switch activation mechanism. These data and computer modeling studies allowed us to hypothesize that the presence of His81 significantly modified the fold of proMMP-26, abolished the functionality of the cysteine-switch motif, and stimulated an alternative intramolecular activation pathway of the proenzyme.