Activity of <b>α</b> 1,6-fucosyltransferase (FUT8) is reduced by depletion of oligosaccharyltransferase subunits

Alpha-1,6-fucosyltransferase (FUT8) transfers fucose to the innermost GlcNAc residue of N-glycans, forming the core fucose structure. Core fucose critically regulates the functions of various glycoproteins and is associated with several diseases, including chronic obstructive pulmonary disease and cancer. However, the regulatory mechanisms of its enzymatic activity and the intracellular localization of FUT8 remain largely unknown. We previously demonstrated that ribophorin I (RPN1), a subunit of the oligosaccharyltransferase (OST) complex, interacts with FUT8 and positively regulates the enzymatic activity of FUT8; however, it remains unclear whether other OST subunits interact with FUT8 and regulate FUT8 activity. In this study, we assessed the enzymatic activity of FUT8 with knockdown of each OST subunit and showed that silencing ribophorin II (RPN2) as well as RPN1 significantly reduced FUT8 activity. By contrast, no significant effect on FUT8 activity was observed by depleting STT3A and STT3B, which are catalytic subunits of OST, suggesting that the regulation of FUT8 activity by OST is irrelevant to the N-glycosylation activity of OST. Furthermore, various OST subunits were detected in anti-FUT8-immunoprecipitates, and FUT8 was detected in STT3s-dependent high molecular weight complexes in native-PAGE, whereas FUT8 at steady state was mainly localized in the Golgi apparatus, distinct from the endoplasmic reticulum localization of OST. These results suggest that FUT8 transiently interacts with OST complexes during transport in cells. Our findings provide insights into both the intracellular regulatory mechanisms of FUT8 activity and some unexplored functions of OST subunits.