ABSTRACT: STT3A and STT3B are the main catalytic subunit of oligosaccharyltransferase (OST-A and OST-B in mammalian cells), which primarily mediate cotranslational and posttranslocational N-linked glycosylation, respectively. To determine the specificity of STT3A and STT3B, we performed proteomics and glycoproteomics analyses to characterize the protein expression and glycosylation in STT3A-knockout (KO), STT3B-KO, and wild-type (WT) HEK293 cells. In total, 3,993 proteins, 4,020 unique N-linked intact glycopeptides (IGPs) and 723 glycosites, representing 401 glycoproteins were identified in KO and WT cells. Deletion of the STT3A gene had a greater impact on the protein expression than deletion of STT3B, especially on glycoproteins. In addition, mannosylated N-glycans from glycoproteins were reduced, while fucosylated N-glycans were increased in STT3A-KO cells. The glycan changes may be caused by the differential expression of glycosyltransferases and the activation of unfolded protein response (UPR) with further analysis of glycan-related enzymes. Interestingly, hyperglycosylated proteins were identified in KO cells. We verified that the hyperglycosylation of ENPL was caused by the ER stress and UPR, which were induced by the deletion of the STT3A. Overall, the specificity of STT3A and STT3B revealed that defects in the OST subunit not only broadly affect N-linked glycosylation of the protein but also affect protein expression.STT3A and STT3B are the main catalytic subunit of oligosaccharyltransferase (OST-A and OST-B in mammalian cells), which primarily mediate cotranslational and posttranslocational N-linked glycosylation, respectively. To determine the specificity of STT3A and STT3B, we performed proteomics and glycoproteomics analyses to characterize the protein expression and glycosylation in STT3A-knockout (KO), STT3B-KO, and wild-type (WT) HEK293 cells. In total, 3,993 proteins, 4,020 unique N-linked intact glycopeptides (IGPs) and 723 glycosites, representing 401 glycoproteins were identified in KO and WT cells. Deletion of the STT3A gene had a greater impact on the protein expression than deletion of STT3B, especially on glycoproteins. In addition, mannosylated N-glycans from glycoproteins were reduced, while fucosylated N-glycans were increased in STT3A-KO cells. The glycan changes may be caused by the differential expression of glycosyltransferases and the activation of unfolded protein response (UPR) with further analysis of glycan-related enzymes. Interestingly, hyperglycosylated proteins were identified in KO cells. We verified that the hyperglycosylation of ENPL was caused by the ER stress and UPR, which were induced by the deletion of the STT3A. Overall, the specificity of STT3A and STT3B revealed that defects in the OST subunit not only broadly affect N-linked glycosylation of the protein but also affect protein expression.
Piaopiao Wen 
