the binding site on the endoplasmic reticulum (ER) membrane for ribosomes bearing nascent polypeptides destined for secretion or incorporation into ER, Golgi apparatus, or plasma membrane. It consists of several proteins (including the ribophorins) oriented with their C-terminal segments on the cytoplasmic ER surface. The proteins bind directly to the large ribosomal subunit, and may also function in translocation of the nascent polypeptide into the ER lumen.