Refined topology model of the STT3/Stt3 protein subunit of the oligosaccharyltransferase complex

The oligosaccharyltransferase complex, localized in the endoplasmic reticulum (ER) of eukaryotic cells, is responsible for the N-linked glycosylation of numerous protein substrates. The membrane protein STT3 is a highly conserved part of the oligosaccharyltransferase and likely contains the active site of the complex. However, understanding the catalytic determinants of this system has been challenging, in part because of a discrepancy in the structural topology of the bacterial versus eukaryotic proteins and incomplete information about the mechanism of membrane integration. Here, we use a glycosylation mapping approach to investigate these questions. We measured the membrane integration efficiency of the mouse STT3-A and yeast Stt3p transmembrane domains (TMDs) and report a refined topology of the N-terminal half of the mouse STT3-A. Our results show that most of the STT3 TMDs are well inserted into the ER membrane on their own or in the presence of the natural flanking residues. However, for the mouse STT3-A hydrophobic domains 4 and 6 and yeast Stt3p domains 2, 3a, 3c, and 6 we measured reduced insertion efficiency into the ER membrane. Furthermore, we mapped the first half of the STT3-A protein, finding two extra hydrophobic domains between the third and the fourthTMD. This result indicates that the eukaryotic STT3 has 13 transmembrane domains, consistent with the structure of the bacterial homolog of STT3 and setting the stage for future combined efforts to interrogate this fascinating system.