THE OLIGOMERIZATION DOMAIN OF THE ASIALOGLYCOPROTEIN RECEPTOR PREFERENTIALLY FORMS 2 2 HETEROTETRAMERS IN-VITRO/

The human hepatic asialoglycoprotein receptor is a noncovalent hetero- oligomer composed of two homologous subunits, H1 and H2, with an as ye t unknown stoichiometry, Ligand specificity and binding affinity depen d on the arrangement of the subunits in the complex. An 80-amino acid segment connecting the transmembrane and the carbohydrate binding doma ins contains heptad repeats characteristic of alpha-helical coned coil structure, We expressed and purified corresponding peptides, H1S and H2S, and confirmed by circular dichroism spectroscopy that they can as sume alpha-helical conformation. Oxidative cross-linking of amino-term inal cysteines generated specific covalent oligomers, indicating that separately H1S forms trimers and H2S tetramers, Upon mixing, covalent heterotetramers mere formed with a preferred stoichiometry of 2 H1S an d 2 H2S peptides. These results suggest that the stalk segments of the receptor subunits oligomerize to constitute an alpha-helical coiled c oil stalk on top of which the carbohydrate binding domains are exposed for ligand binding, We propose that the functional asialoglycoprotein receptor is a 2:2 heterotetramer.