Membrane transport in rat liver endocytic pathways: Preparation, biochemical properties and functional roles of hepatic endosomes

The endocytic compartment has emerged as a major regulator of the uptake and processing of circulating ligands, and has been extensively studied during the last decade. In this work, the polypeptides of the three endosomal fractions: compartment of uncoupling receptors and ligands (CURL), multivesicular bodies (MVB) and receptor recycling compartment (RRC), isolated from livers of estradiol-treated rats, were analyzed by two-dimensional gel electrophoresis. Silver-stained gels revealed that although the three endosomal fractions shared a generally similar pattern of approximately 120 components, qualitative and quantitative differences between the three endocytic fractions could be demonstrated. The polypeptide composition of the bile was also studied and compared with ligands and proteins identified in the different endosomal fractions. One- and two-dimensional gel electrophoresis and Western blotting were used to investigate the protein composition of the three isolated endocytic fractions and 39 proteins were identified. The distribution of identified receptors, ligands and structural proteins among the three endosomal fractions was in agreement with their expected functionalities and with the different endocytic pathways in the hepatocyte.