Hepatocytes must transport newly synthesized apical membrane proteins from the basolateral to the apical plasma membrane. Our earlier morphological study showed that the apical proteins share a late (subapical) part of the transcytotic pathway with the well characterized polymeric immunoglobulin A receptor (Barr, V. A., and Hubbard, A. L.(1993) Gastroenterology 105, 554-571). Starting with crude microsomes from the livers of [S]methionine-labeled rats, we sequentially immunoadsorbed first vesicles containing the endocytic asialoglycoprotein receptor and then (from the depleted supernatant) vesicles containing the polymeric IgA receptor. Biochemical characterization indicated that early basolateral and late endosomes were present in the first population but not in the second. Neither Golgi-, apical plasma membrane (PM)-, nor basolateral PM-derived vesicles were significant contaminants of either population. Both vesicle populations contained S-labeled receptor andS-labeled-dipeptidyl peptidase IV. Importantly, the elevated relative specific activity of the dipeptidyl peptidase (% of S-labeled/% immunoblotted) in the second population indicated that these vesicles must transport newly synthesized dipeptidyl peptidase IV. A distinct kind of vesicle was immunoadsorbed from a ``carrier-vesicle fraction''; surprisingly, these vesicles contained little S-receptor and virtually no dipeptidyl peptidase IV. These results, together with previous kinetic data from in vivo experiments, are consistent with a computer-generated model predicting that newly synthesized dipeptidyl peptidase IV is delivered to basolateral endosomes, which also contain newly synthesized polymeric immunoglobulin A receptor. The two proteins are then transcytosed together to the subapical region.

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