Role of the COOH-terminal domains of meprin A in folding, secretion, and activity of the metalloendopeptidase

Secreted forms of the cu subunit of recombinant mouse meprin A include an NH2-terminal prosequence, a catalytic domain, and three COOH-terminal domains designated as MAM (meprin, A-5 protein, receptor protein-tyrosine phosphatase mu), MATH (meprin and TRAF homology), and AM (after MATH). In this study, the importance of these COOH-terminal domains for biosynthesis of secreted, activable forms of the protease was investigated. Transcripts of the meprin subunit truncated after the protease (alpha(1-275)), MAM (alpha(1-452)), and MATH (alpha(1-528)) domains or with individual domains deleted (Delta MAM, Delta MATH, and Delta AM), were transfected into human embryonic kidney 293 cells. The wild-type subunit, Delta MATH, Delta AM, alpha(1-452), and alpha(1-528) were secreted into the media, although the Delta AM mutant was secreted at very low levels. The Delta MATH and alpha(1-452) mutants were not activable by Limited proteolysis. The alpha(1-528) mutant was as active as wild-type meprin cu against a bradykinin substrate, but had no activity against azocasein, and it, as all other mutants, was more vulnerable to extensive degradation by proteases than the wild-type protein. Pulse-chase experiments revealed that the Delta MAM and alpha(1-275) mutants were rapidly degraded within cells. Treatment with lactacystin, a specific inhibitor of the proteasome, significantly decreased the degradation, indicating that the mutants lacking the MAM domain are degraded by the proteasome as misfolded proteins. These results indicate that the MAM domain is necessary for correct folding and transport through the secretory pathway, the MATH domain is required for folding of an activable zymogen, and the AM domain is important for activity against proteins and efficient secretion of the protein. The work demonstrates the interdependence of the domains for correct folding of an activable, stable, mature enzyme.