HOMOTETRAMERIC AND HETEROTETRAMERIC FORMS OF THE MEMBRANE-BOUND METALLOENDOPEPTIDASES MEPRIN-A AND MEPRIN-B

Meprin A and meprin B are disulfide-linked, tetrameric metalloendopeptidases in renal brush border membranes. Meprin A contains 90-kDa subunits (α subunits) and is expressed in random-bred and some inbred strains of mice. Meprin B contains subunits of 110 kDa (β subunits) in situ, and the enzyme from C3H mice, a strain that does not express α subunits, has been characterized. Evidence from this and previous studies indicate that β subunits are expressed in all mouse strains. The tetrameric organization of these meprins was examined in brush border membrane fractions from a random-bred strain (ICR) and two inbred strains of mice (C57BL/6 and C3H/He). Lectin blotting using biotinylated concanavalin A revealed that membranes from the random-bred strain contained three oligomeric complexes of approximately 390, 440, and 490 kDa as determined after sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) in the absence of reducing agents. The subunits in all three oligomers were linked by disulfide bridges. Western blotting using an anti-α monoclonal antibody indicated that α subunits (90 kDa) were present in the 390- and 440-kDa complexes. Western blotting with a polyclonal antibody specific for β subunits (110 kDa) indicated the presence of these subunits in the 440-and 490-kDa complexes. Electroelution of the individual oligomers followed by SDS-PAGE under reducing conditions confirmed that the 390- and 490-kDa molecules are homotetramers of α and β subunits, respectively, and that the 440-kDa molecule is a heterotetramer consisting of disulfide-bridged α and β subunits. C57BL/6 mice expressed both α and β subunits and contained tetramers composed of α4 and α2β2. C3H/He mice expressed only the 110-kDa β subunits and the β4 oligomer. This type of multimeric organization of disulfide-linked subunits is unique for the known endopeptidases. © 1991.