Major and minor forms of the rat liver asialoglycoprotein receptor are independent galactose-binding proteins. Primary structure and glycosylation heterogeneity of minor receptor forms.

Preparations of the rat liver asialoglycoprotein receptor (rat hepatic lectin, RHL), which is responsible for the selective uptake of partially deglycosylated serum glycoproteins, have been found to contain multiple polypeptide species. A method has been developed for separating the predominant species (RHL-1) from the minor species (RHL-2/3) using conditions in which RHL-1 retains its galactose-binding activity. Endoglycosidase digestion and lectin blotting have been utilized to demonstrate that RHL-2 and RHL-3 differ by the presence of different carbohydrate structures attached to a common peptide backbone. Antisera specific for RHL-1 and RHL-2/3 have been prepared and utilized to analyze the results of cross-linking experiments performed on purified receptor and hepatocyte membranes. The results show that RHL-1 and RHL-2/3 polypeptides are each associated into homooligomers but are physically unlinked to each other. The structure of the RHL-2/3 polypeptide has been established by protein and cDNA sequence analysis, which reveals that this protein is homologous to RHL-1 throughout its length but contains one major insertion of 18 amino acids near its NH2 terminus. The COOH-terminal portion of the RHL-2/3 polypeptide has been demonstrated to contain a galactose-recognition domain by expression in an in vitro transcription/translation system. The results of these experiments indicate that RHL-1 and RHL-2/3 polypeptides are self-associated into two distinct molecules, each of which has galactose-binding activity.