Primary structure of the rat liver asialoglycoprotein receptor. Structural evidence for multiple polypeptide species

When preparations of rat liver receptor for asialoglycoproteins (rat hepatic lectin, (RHL] are examined by dodecyl sulfate-polyacrylamide gel electrophoresis, multiple polypeptide species are found to be present. The predominant polypeptide has an apparent molecular weight of 41,500 (RHL-1), while two less abundant species appear to be of higher molecular weight (49,000 (RHL-2) and 54,000 (RHL-3]. When the several polypeptides are separated and treated with BrCN, the two minor species are found to share at least one large fragment, while the RHL-1 species gives rise to a completely different set of BrCN peptides. All of the BrCN fragments of the major species and the large common fragment from RHL-2 and RHL-3 have been isolated. These fragments serve as the basis for the complete sequence determination of RHL-1. The complete sequence is 283 residues long, although 20% of the protein as isolated is missing the first 2 residues at the NH2 terminus. The overall arrangement of the polypeptide is similar to the chicken receptor for asialoagalactoglycoproteins; it consists of an NH2-terminal stretch of hydrophilic amino acids, a segment of about 30 uncharged residues, and a COOH-terminal portion which contains three oligosaccharide attachment sites. When the COOH terminus of the rat liver receptor is aligned with the corresponding portions of the chicken liver receptor, the two proteins show 28% identity. Little identity is seen near the NH2 terminus. Sequence homology between residues 50-79 and residues 121-150 of the rat receptor suggests that the additional length of this protein compared with the chicken protein may be due to the presence of a duplicated segment within the rat receptor. The complete sequence of the BrCN fragment common to the two minor species has also been determined; this 101-residue sequence is 53% identical with the COOH-terminal sequence of RHL-1. Since these minor species have a primary structure distinct from RHL-1, there must be at least two genes coding for receptor polypeptides. RHL-2 and RHL-3 may differ in their extent of posttranslational modification.