Proteolytic processing of rat liver membrane secretory component. Cleavage activity is localized to bile canalicular membranes

Membrane secretory component (mSC) mediates the transcellular movement of polymeric IgA from the sinusoidal to the bile canalicular surface of rat hepatocytes. Prior to or concomitant with arrival at the bile canalicular membrane, mSC is cleaved, producing a soluble proteolytic fragment (fSC) which is released into the bile. Conversion of mSC to fSC occurs at the cell surface of cultured rat hepatocytes (Musil, L.S., and Baenziger, J.U. (1987) J. Cell Biol. 104, 1725-1733), suggesting that vectorial release of fSC into bile in vivo may reflect localization of a mSC-specific protease to bile canalicular membranes. We have established a reconstituted system to examine the process of specific cleavage of mSC to yield fSC and to characterize the protease activity responsible. A membrane fraction highly enriched for endocytic vesicles was found to contain ~90% of the [³⁵S]Cys-mSC from metabolically labeled rat liver slices but only 5% of the cellular protein. No cleavage activity was present in these vesicles. Highly enriched bile canalicular membranes were able to mediate cleavage of metabolically labeled mSC to a fragment indistinguishable from authentic fSC. In the absence of nonionic detergent, cleavage was dependent on the presence of polyethylene glycol, presumably to mediate fusion of mSC-enriched membranes with bile canalicular membranes. Following solubilization with nonionic detergent, cleavage was no longer dependent on the addition of polyethylene glycol. Cleavage of mSC was not observed with either intact or detergent-solubilized sinusoidal, microsomal, or lysosomal membranes. We have thus identified a proteolytic activity associated with bile canalicular membranes which has the properties of a membrane protein and is likely to be responsible for production of fSC in vivo. Its highly restricted localization to the bile canalicular membrane would account for the vectorial release of fSC into the bile.