Determinants of oligomeric structure in the chicken liver glycoprotein receptor

The oligomeric state of the chicken liver receptor (chicken hepatic lectin), which mediates endocytosis of glycoproteins terminating with N-acetylglucosamine, has been investigated using physical methods as well as chemical cross-linking. Receptor isolated from liver and from transfected rat fibroblasts expressing the full-length polypeptide is a homotrimer immediately following solubilization in non-ionic detergent, but forms the previously observed hexamer during purification. These results are most consistent with the presence of a trimer of receptor polypeptides in liver membranes and in transfected cells. Analysis of truncated receptors reveals that the C-terminal extracellular portion of this type-II transmembrane protein does not form stable oligomers when isolated from the membrane anchor and cytoplasmic tail. The behaviour of chimeric receptors, in which the cytoplasmic tail of the glycoprotein receptor is replaced with the corresponding segments of rat liver asialoglycoprotein receptor or the beta-subunit of Na+,K(+)-ATPase, or with unrelated sequences from globin, indicates that the cytoplasmic tail influences oligomer stability. Replacement of N-terminal portions of the receptor with corresponding segments of influenza virus neuraminidase results in formation of tetramers, suggesting that the membrane anchor and flanking sequences are important determinants of oligomer formation.