CHARACTERIZATION OF CLONED RAT-LIVER NA+-BILE ACID COTRANSPORTER USING PEPTIDE AND FUSION PROTEIN ANTIBODIES

A cDNA encoding a rat liver Na+-bile acid cotransporter (Ntcp) has recently been cloned (Hagenbuch, B., B. Steiger, M. Fouget, H. Lubbert, and P. J. Meier. Proc. Natl. Acad. Sci. USA 88: 10629, 1991) using expression cloning in Xenopus laevis oocytes. Although the open reading frame coded for a protein of 39 kDa, in vitro translation experiments produced a 35-kDa protein which increased to a product of 41 kDa after glycosylation by pancreatic microsomes. To more clearly characterize the native protein in rat liver, we have raised antipeptide and anti-fusion protein antibodies to the COOH-terminal part of the cloned transporter. On Western blot analysis both antisera but not preimmune serum specifically detected a protein of similar to 50 kDa in isolated rat liver basolateral plasma membranes (BLPM). The reactivity was abolished when the antiserum was preincubated with the synthetic alpha-337 peptide. Deglycosylation of BLPM with N-glycanase followed by antibody probing led to decrease of the molecular mass to 34.5 kDa, suggesting that the protein is N-glycosylated in vivo. Two-dimensional immunoblotting indicated that the Ntcp protein had an isoelectric point of similar to 6.0. The antibody did not react with any proteins in rat ileal and kidney cortex brush-border membranes, human liver basolateral plasma membranes, or rat hepatoma tissue culture cell homogenates. Immunofluorescence localization studies with both antibodies revealed specific staining of the sinusoidal membrane domain but not of intracellular or bile canalicular membranes. Moreover, there was no acinar gradient in the pattern of staining. In conclusion, antibodies specific to rat liver Ntcp have been produced which recognize a 50-kDa protein in BLPM. The transport protein was localized exclusively to the basolateral domain and was evenly distributed across the hepatic lobule.