DETERMINANTS OF OLIGOMERIC STRUCTURE IN THE CHICKEN LIVER GLYCOPROTEIN RECEPTOR

The oligomeric state of the chicken liver receptor (chicken hepatic le ctin), which mediates endocytosis of glycoproteins terminating with N- acetylglucosamine, has been investigated using physical methods as wel l as chemical cross-linking. Receptor isolated from liver and from tra nsfected rat fibroblasts expressing the full-length polypeptide is a h omotrimer 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 w hen isolated from the membrane anchor and cytoplasmic tail. The behavi our of chimeric receptors, in which the cytoplasmic tail of the glycop rotein receptor is replaced with the corresponding segments of rat liv er asialoglycoprotein receptor or the 8-subunit of Na+,K+-ATPase, or w ith unrelated sequences from globin, indicates that the cytoplasmic ta il influences oligomer stability. Replacement of N-terminal portions o f the receptor with corresponding segments of influenza virus neuramin idase results in formation of tetramers, suggesting that the membrane anchor and flanking sequences are important determinants of oligomer f ormation.