Membrane association and oligomeric organization of the alpha and beta subunits of mouse meprin A.

Meprins are oligomeric cell surface metalloproteinases of the "astacin family." They consist of two types of subunits (alpha and beta), which are evolutionarily related and whose cDNA sequences predict a similar domain structure. The present work shows that reducing agents solubilized meprin alpha subunits (approximately 90%), but not beta subunits, from mouse kidney brush border membranes. In addition, immunoblotting of membranes or purified meprins with an antibody raised to the alpha subunit epidermal growth factor-like domain, predicted to be near the COOH terminus from the cDNA-deduced amino acid sequence, indicated that this domain is not present in the mature alpha subunit. By contrast, an epitope predicted to be near the COOH terminus of the beta subunit was present in the mature form of beta. When meprins were solubilized from brush border membranes by papain, the size of the alpha subunit (approximately 90 kDa) did not change, while the beta subunit decreased from 110 to 90 kDa with concomitant loss of the COOH-terminal epitope. These data indicate that beta is a type I transmembrane protein, while alpha does not transverse the membrane and its association is dependent on disulfide bonds. The oligomeric organization of purified meprin A (EC 3.4.24.18), examined by sedimentation equilibrium analysis and native gradient gel electrophoresis, is that of disulfide-bridged dimers which aggregate noncovalently to form higher molecular weight complexes, predominantly tetramers. Western blotting of ICR kidney brush border membrane proteins identified alpha 2 homodimers and alpha beta heterodimers. Treatment of mouse or rat kidney brush border membranes with 7 M urea solubilized alpha 2, but not alpha beta dimers. Thus, the mature alpha subunit exists in alpha 2 and alpha beta disulfide-linked dimers which form tetramers, and the alpha 2 homodimers associate with the membrane through noncovalent interactions with alpha beta.