Cleavage of membrane secretory component to soluble secretory component occurs on the cell surface of rat hepatocyte monolayers.

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Abstract
Rat liver secretory component is synthesized as an integral protein (mSC) and cleaved to an 80-kD soluble form (fSC) sometime during transcellular transport from the sinusoidal to the bile canalicular plasma membrane domain of hepatocytes. We have used 24-h monolayer cultures of rat hepatocytes to characterize the conversion of mSC to fSC. Cleavage of mSC in cultured hepatocytes is inhibited by the thiol protease inhibitors leupeptin, antipain, and E-64, but not by other inhibitors, including diisopropylfluorophosphate, pepstatin, N-ethylmalemide, p-chloromercuribenzoic acid, and chloroquine. Leupeptin-mediated inhibition of cleavage is concentration dependent and reversible. In the presence or absence of leupeptin, only 10-20% of mSC is accessible at the cell surface. To characterize the behavior of surface as opposed to intracellular mSC, cell surface mSC was labeled with 125I by lactoperoxidase-catalyzed iodination at 4.degree. C. Cell surface 125I-mSC was converted to extracellular fSC at 4.degree. C in the absence of detectable internalization. Cleavage was inhibited by leupeptin and by anti-secretory component antiserum. Cleavage also occurred at 4.degree.C after cell disruption. In contrast, 125I-mSC that had been internalized from the cell surafce was not converted to fSC at 4.degree. C in either intact or disrupted cells. Hepatocytes metabolically labeled with [35S]cys also released small quantities of fSC into the medium at 4.degree. C. The properties of fSC production indicate that cleavage occurs on the surface of cultured rat hepatocytes and not intracellularly. Other features of the cleavage reaction suggest that the mSC-cleaving protease is segregated from the majority of cell surface mSC, possibly within a specialized plasma membrane domain.