The major subunit of the rat asialoglycoprotein receptor can function alone as a receptor

Mammalian hepatic asialoglycoprotein receptors (ASGP-R) are composed of two unique, but closely related polypeptides, which in the rat are designated rat hepatic lectins 1 and 2/3 (RHL 1, RHL 2/3). Despite numerous studies, the composition of a functional ASGP-R has remained unclear. We examined this question in rat hepatoma tissue culture (HTC) cells (which lack endogenous ASGP-R) that were co-transfected with cDNAs for both RHL 1 and RHL 2/3. The original population was cloned, but derivatives were unstable. We therefore used fluorescence-activated cell sorting to separate a subpopulation of cells (positive) that specifically endocytosed fluoresceinated asialoorosomucoid (ASOR) from one that did not (negative). We then used indirect immunofluorescence with polypeptide-specific ASGP-R antibodies, immunoanalysis, and binding and uptake studies with two Gal ligands (ASOR and NAc-galactosylated poly-L-lysine (Gal-Lys)) to further define the ASGP-R status in these two populations. As reported by others, we found that expression of both RHL 1 and RHL 2/3 in the positive cells resulted in binding, uptake and degradation of ASOR, the most commonly used ASGP-R ligand. The negative cells expressed only RHL 1 and neither bound nor processed ASOR. However, the presence of RHL 1 was sufficient for specific high affinity binding and processing of the synthetic ligand, Gal-Lys, by negative cells. These results show that RHL 1 can function as an ASGP-R, given a highly galactosylated ligand, and that RHL 2/3 must play an important role in the organization of native ASGP-R in the membrane.