Biosynthesis and insertion of a hepatic binding protein specific for asialoglycoproteins into endoplasmic reticulum...

Biosynthesis of a hepatic binding protein specific for asialoglycoproteins and its subsequent insertion into microsomal membrane were studied by using antibody monospecific for the binding protein. 125I-Labeled antibody binds much more preferentially with membrane-bound ribosomes than with free ribosomes, and nascent binding protein labeled with [3H]puromycin was detected exclusively on tightly membrane-bound ribosomes, which can be detached from the membrane by puromycin treatment in the presence of high salt buffer. When rough microsomes labeled in vivo with 14C-amino acid mixture were digested with protease, nascent binding protein was effectively protected from the digestion like nascent albumin (approximately 90%). When rough microsomes labeled in vitro with [3H]puromycin were digested with protease, the degree of protection of albumin was again approximately 90%, whereas that of the binding protein was only approximately 50%. The carbohydrate moieties of the binding protein and the bulk of glycoproteins in the microsomes labeled in vivo with [3H]glycosamine and [3H]mannose were also effectively protected from the protease digestion (approximately 90%). These results indicate that the binding protein is exclusively synthesized on the membrane-bound ribosomes and spans the microsomal membrane probably exposing the carboxyl-terminal segment on the cytoplasmic surface and the amino-terminal segment charged with carbohydrate moieties on the luminal surface, respectively.