Expression of meprin subunit precursors. Membrane anchoring through the beta subunit and mechanism of zymogen activation.

The biosynthesis of the membrane-bound metalloendopeptidase meprin was studied after the introduction of cDNAs encoding the rat meprin alpha and beta subunits into human 293 cells. When expressed individually the meprin alpha subunit was found to be primarily secreted into the culture medium, while the beta subunit was determined to be an integral membrane protein. Coexpression of the alpha and beta subunits results in the localization of both subunits to the plasma membrane. In this case the alpha subunit is specifically released from the cell surface by dithiothreitol, indicating the alpha subunit is associated with the membrane via disulfide bond(s) to the beta subunit. Meprin expressed in 293 cells is similar to the rat kidney enzyme in that it forms disulfide-linked dimers and has a similar pattern of glycosylation. However, the expressed protein displayed no detectable peptidase activity against four meprin substrates. Processing of the alpha subunit was followed after the introduction of sequences coding for the human c-myc peptide epitope EQKLISEEDL into its cDNA in the region of its prosequence and at the COOH terminus. Detection of the resulting proteins using a monoclonal antibody specific for the c-myc epitope indicates the alpha subunit is processed at its COOH terminus but retains the prosequence which is absent from the enzyme purified from rat kidney. Limited trypsin digestion of meprin precursors expressed in 293 cells results in both the activation of the enzyme and the removal of the prosequence. This result supports the hypothesis of Bode et al. (Bode W., Gomis-Ruth, F. X., Huber, R., Zwilling, R., and Stocker, W. (1992) Nature 358, 164-167) that meprin and other astacin family proteases require removal of NH2-terminal prosequences at the junction of the astacin protease domain for zymogen activation. Trypsin-activated meprin was assayed with the protein substrate azocasein and three synthetic peptide substrates. The membrane-bound beta subunit was found to be more active than the secreted alpha subunit against azocasein but much less active toward the synthetic peptide substrates.