Oligosaccharyltransferase (OST) catalyzes the cotranslational transfer of high-mannose sugars to nascent polypeptides during N-linked glycosylation in the rough endoplasmic reticulum lumen. Nine OST subunits have been identified in yeast. However, the composition and organization of mammalian OST remain unclear. Using two-dimensional Blue Native polyacrylamide gel electrophoresis/sodium dodecyl sulfate-polyacrylamide gel electrophoresis and mass spectrometry, we now demonstrate that mammalian OST can be isolated from solubilized, actively engaged ribosomes as multiple distinct protein complexes that range in size from ∼500 to 700 kDa. These complexes exhibit different ribosome affinities and subunit compositions. The major complex, OSTC I, had an apparent size of ∼500 kDa and was readily released from ribosome translocon complexes after puromycin treatment under physiological salt conditions. Two additional complexes were released only after treatment with high salt: OSTCII (∼600 kDa) and OSTCIII (∼700 kDa). Both remained stably associated with heterotrimeric Sec61αβγ, while OSTCIII also contained the tetrameric TRAP complex. All known mammalian OST subunits (STT3-A, ribophorin I, ribophorin II, OST48, and DAD1) were present in all complexes. In addition, two previously uncharacterized proteins were also copurified with OST. Mass spectrometry identified a 17 kDa protein as DC2 which is weakly homologous to the C-terminal half of yeast Ost3p and Ostop. The second protein (14 kDa) was tentatively identified as keratinocyte-associated protein 2 (KCP2) and has no previously known function. Our results identify two potential new subunits of mammalian OST and demonstrate a remarkable heterogeneity in OST composition that may reflect a means for controlling nascent chain glycosylation.
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