Interaction of FAM5C with UDP-glucose:glycoprotein glucosyltransferase 1 (UGGT1): Implication of N-glycosylation in FAM5C secretion.

N-glycosylation of proteins is important for protein folding and function. We have recently reported that FAM5C/BRINP3 contributes to the tumor necrosis factor-α-induced expression of leukocyte adhesion molecules in vascular endothelial cells (ECs). However, regulatory mechanism of the FAM5C biosynthesis is poorly understood. Co-immunoprecipitation assay revealed the interaction of FAM5C with UDP-glucose:glycoprotein glucosyltransferase 1 (UGGT1), a glycoprotein folding-sensor enzyme. FAM5C ectopically expressed in HEK293 cells was localized to the endoplasmic reticulum and co-localized with endogenously expressed UGGT1. Molecular size of FAM5C was reduced by treatment with N-glycosidase F and in FAM5C-expressing cells cultured in the presence of the N-glycosylation inhibitor tunicamycin. FAM5C was secreted by the cells and the secretion of FAM5C was blocked by tunicamycin. Among six potential N-glycosylation sites, the potential site at Asn ¹⁶⁸ was not N-glycosylated, and Asn ³³⁷ , Asn ⁴⁵⁶ , Asn ⁵⁶² , Asn ⁶⁰⁹ , and Asn ⁶⁴¹ mutants were poorly secreted by the cells. These results demonstrated that FAM5C is an N-glycosylated protein and N-glycosylation is necessary for the secretion of FAM5C.
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