Protein N-glycosylation is widespread among biological systems, and the fundamental process of transferring a lipid-linked glycan to suitable asparagine residues of newly synthesized proteins occurs in both prokaryotes and eukaryotes. The core reaction is mediated by Stt3p family members, and in many organisms this component alone is sufficient to constitute the so called oligosaccharyltransferase (OST). However, eukaryotes typically have a more elaborate OST with several additional subunits of poorly defined function. In the mammalian OST complex one such subunit, ribophorin I, is proposed to facilitate the N-glycosylation of certain precursors during their biogenesis at the endoplasmic reticulum. Here, we use cell culture models to show that ribophorin I depletion results in substrate-specific defects in N-glycosylation, clearly establishing a defined physiological role for ribophorin I. To address the molecular mechanism of ribophorin I function, a cross-linking approach was used to explore the environment of nascent glycoproteins during the N-glycosylation reaction. We show for the first time that ribophorin I can regulate the delivery of precursor proteins to the OST complex by capturing substrates and presenting them to the catalytic core.