IMMUNOADSORPTION OF HEPATIC VESICLES CARRYING NEWLY SYNTHESIZED DIPEPTIDYL PEPTIDASE-IV AND POLYMERIC IGA RECEPTOR

Hepatocytes must transport newly synthesized apical membrane proteins from the basolateral to the apical plasma membrane, Our earlier morpho logical study showed that the apical proteins share a late (subapical) part of the transcytotic pathway with the well characterized polymeri c immunoglobulin A receptor (Barr, V. A., and Hubbard, A. L. (1993) Ga stroenterology 105, 554-571), Starting with crude microsomes from the livers of [S-35]methionine-labeled rats, we sequentially immuno-adsorb ed first vesicles containing the endocytic asialoglycoprotein receptor and then (from the depleted supernatant) vesicles containing the poly meric IgA receptor, Biochemical characterization indicated that early basolateral and late endosomes were present in the first population bu t not in the second. Neither Golgi, apical plasma membrane (PM)-, nor basolateral PM-derived vesicles were significant contaminants of eithe r population. Both vesicle populations contained S-35-labeled receptor and S-35-labeled-dipeptidyl peptidase TV, Importantly, the elevated r elative specific activity of the dipeptidyl peptidase (% of S-35-label ed/% immunoblotted) in the second population indicated that these vesi cles must transport newly synthesized dipeptidyl peptidase IV. A disti nct kind of vesicle was immunoadsorbed from a ''carrier-vesicle fracti on''; surprisingly, these vesicles contained little S-35-receptor and virtually no dipeptidyl peptidase IV. These results, together with pre vious kinetic data from in vivo experiments, are consistent with a com puter-generated model predicting that newly synthesized dipeptidyl pep tidase IV is delivered to basolateral endosomes, which also contain ne wly synthesized polymeric immunoglobulin A receptor. The two proteins are then transcytosed together to the subapical region.