YIPF1, YIPF2, and YIPF6 are medial-/trans-Golgi and trans-Golgi network-localized Yip domain family proteins, which play a role in the Golgi reassembly and glycan synthesis

In this study, we attempted to explore the function of three uncharacterized mammalian homologs of yeast Yip domain family proteins-YIPF6, a homolog of Yip1p, and YIPF1 and YIPF2, which are homologs of Yif1p. Immunofluorescence staining revealed that YIPF1, YIPF2, and YIPF6 mainly localize in the medial-/transGolgi and also partially in the trans-Golgi network (TGN). On treatment with brefeldin A (BFA), the homologs co-migrated partly with medial-/trans-Golgi markers and also with a TGN marker in earlier time point, but finally redistributed within cytoplasmic punctate structures that were distinct from medial-/trans-Golgi and the TGN markers. YIPF6 formed a stable complex separately with YIPF1 and YIPF2, and knockdown of YIPF6 reduced YIPF1 and YIPF2 levels. These results suggest that YIPF6 forms complexes with YIPF1 and YIPF2 for their stable expression and localization within the Golgi apparatus. Knockdown experiments showed that YIPF1 and YIPF2, by contrast, are not necessary for the expression and localization of YIPF6. The structure of the Golgi apparatus and its disassembly after BFA treatment were not significantly affected by the knockdown of YIPF1, YIPF2, or YIPF6. However, reassembly of the Golgi apparatus after the removal of BFA was markedly delayed by the knockdown of YIPF1 and YIPF2, but not by that of YIPF6. These results strongly suggest that free YIPF6 after disassociating with YIPFI and YIPF2 interferes with the reassembly of the Golgi apparatus. Knockdown of YIPF1 and YIPF2, but not that of YIPF6, also reduced intracellular glycans in HT-29 cells. Thus, we confirmed that YIPF1, YIPF2, and YIPF6 play a significant role in supporting normal glycan synthesis.