Abstract
Following initiation of translocation across the membrane of the endoplasmic reticulum via the translocon, polypeptide chains are N-glycosylated by the oligosaccharyl transferase (OT) enzyme complex. Translocation and N-glycosylation are concurrent events and would be expected to require juxtaposition of the translocon and the OT complex. To determine whether any of the subunits of the OT complex and translocon mediate interactions between the two complexes, we initiated a systematic study in budding yeast using the split-ubiquitin approach. Interestingly, the OT subunit Stt3p was found to interact only with Sec61p, whereas another OT subunit, Ost4p, was found to interact with all three components of the translocon, Sec61p, Sbh1p, and Sss1p. The OT subunit Wbp1p was found to interact very strongly with Sec61p and Sbh1p and weakly with Sss1p. Other OT subunits, Ost1p, Ost2p, and Swp1p were found to interact with Sec61p and either Sbh1p or Sss1p. Ost3p exhibited a weak interaction with Sec61p and Sbh1p, whereas Ost5p and Ost6p interacted very weakly with Sec61p and failed to interact with Sbh1p or Sss1p. We were able to confirm these split-ubiquitin findings by a chemical cross-linking technique. Based on our findings using these two techniques, we conclude that the association of these two complexes is stabilized via multiple protein-protein contacts. Based on extrapolation of the structural parameters of the crystal structure of the prokaryotic Sec complex to the eukaryotic complex, we propose a working model to understand the organization of the translocon-OT supercomplex.
Highlights
Based on extrapolation of the structural parameters of the crystal structure of the prokaryotic Sec complex to the eukaryotic complex, we propose a working model to understand the organization of the translocon-oligosaccharyl transferase (OT) supercomplex
Most secretory and membrane proteins are synthesized in the cytosol and are translocated across or integrated into the endoplasmic reticulum (ER)1 membrane via a complex of proteins called the translocon
Split-Ubiquitin Approach to Study the Interaction between the Subunits of the Translocon and OT—Using the split-ubiquitin approach, which is being widely used to study protein-protein interactions [26, 32,33,34,35,36], we analyzed the interac
Summary
Most secretory and membrane proteins are synthesized in the cytosol and are translocated across or integrated into the endoplasmic reticulum (ER) membrane via a complex of proteins called the translocon (for a review on translocation, see Ref. 1). Translocation and N-glycosylation are believed to be concomitant events, because Nglycosylation sites can occur almost anywhere along the length of the newly synthesized polypeptide chain. It appears that coordination between the two processes is necessary; otherwise the glycosylatable sites might be buried during the process of protein folding. Nascent chains undergoing translocation or integration at the ER membrane were found to cross-link to the STT3 subunit of the OT complex, suggesting a close proximity of the OT complex to the translocon [25]
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