Characterization of the ribosomal binding site in rat liver rough microsomes: ribophorins I and II, two integral membrane proteins related to ribosome binding.

Rat liver rough endoplasmic reticulum membranes (ER) contain two characteristic transmembrane glycoproteins which have been designated ribophorins I and II and are absent from smooth ER membranes. These proteins (MW 65,000 and 63,000 respectively) are related to the binding sites for ribosomes, as suggested by the following findings: i) The ribophorin content of the rough ER membranes corresponds stoichiometrically to the number of bound ribosomes; ii) ribophorins are quantitatively recovered with the bound polysomes after most other ER membrane proteins are dissolved with the nonionic detergent Kyro EOB; iii) in intact rough microsomes ribophorins can be cross-linked chemically to the ribosomes and therefore are in close proximity to them. Treatment of rough microsomes with a low Triton-X-100 concentration leads to the lateral displacement of ribosomes on the microsomal surface and to the formation of aggregates of bound ribosomes in areas of membranes which frequently invaginate into the microsomal lumen. Subfractionation of Triton-treated microsomes containing invaginations led to the recovery of smooth and "rough-inverted" vesicles. Ribophorins were present only in the latter fraction, indicating that both proteins are displaced together with the ribosomes when these aggregate without detaching. Measurements of the ribosome-binding capacity of rough and smooth microsomal membranes reconstituted after solubilization with detergents suggest that ribophorins are necessary for in vitro ribosome binding. Ribophorin-like proteins were found in rough microsomes obtained from secretory tissues of several animal species. The two proteins present in rat lacrimal gland microsomes have the same mobility as hepatocyte ribophorins and cross-react with antisera against them.