Heterologous expression and biophysical characterization of soluble oligosaccharyl transferase subunits

Bacillus anthracis tagO Is Required for Vegetative Growth and Secondary Cell Wall Polysaccharide Synthesis

Bacillus anthracis elaborates a linear secondary cell wall polysaccharide (SCWP) that retains surface (S)-layer and associated proteins via their S-layer homology (SLH) domains. The SCWP is comprised of trisaccharide repeats [→4)-β-ManNAc-(1→4)-β-GlcNAc-(1→6)-α-GlcNAc-(1→] and tethered via acid-labile phosphodiester bonds to peptidoglycan. Earlier work identified UDP-GlcNAc 2-epimerases GneY (BAS5048) and GneZ (BAS5117), which act as catalysts of ManNAc synthesis, as well as a polysaccharide deacetylase (BAS5051), as factors contributing to SCWP synthesis. Here, we show that tagO (BAS5050), which encodes a UDP-N-acetylglucosamine:undecaprenyl-P N-acetylglucosaminyl 1-P transferase, the enzyme that initiates the synthesis of murein linkage units, is required for B. anthracis SCWP synthesis and S-layer assembly. Similar to gneY-gneZ mutants, B. anthracis strains lacking tagO cannot maintain cell shape or support vegetative growth. In contrast, mutations in BAS5051 do not affect B. anthracis cell shape, vegetative growth, SCWP synthesis, or S-layer assembly. These data suggest that TagO-mediated murein linkage unit assembly supports SCWP synthesis and attachment to the peptidoglycan via acid-labile phosphodiester bonds. Further, B. anthracis variants unable to synthesize SCWP trisaccharide repeats cannot sustain cell shape and vegetative growth.

IMPORTANCE

Bacillus anthracis elaborates an SCWP to support vegetative growth and envelope assembly. Here, we show that some, but not all, SCWP synthesis is dependent on tagO-derived murein linkage units and subsequent attachment of SCWP to peptidoglycan. The data implicate secondary polymer modifications of peptidoglycan and subcellular distributions as a key feature of the cell cycle in Gram-positive bacteria and establish foundations for work on the molecular functions of the SCWP and on inhibitors with antibiotic attributes.

Structure of the oligosaccharyl transferase complex at 12 A resolution

Oligosaccharyl transferase (OT) catalyzes the transfer of a lipid-linked oligosaccharide to the nascent polypeptide emerging from the translocon. Currently, there is no structural information on the membrane-embedded OT complex, which consists of eight different polypeptide chains. We report a 12 A resolution cryo-electron microscopy structure of OT from yeast. We mapped the locations of four essential OT subunits through a maltose-binding protein fusion strategy. OT was found to have a large domain in the lumenal side of endoplasmic reticulum where the catalysis occurs. The lumenal domain mainly comprises the catalytic Stt3p, the donor substrate-recognizing Wbp1p, and the acceptor substrate-recognizing Ost1p. A prominent groove was observed between these subunits, and we propose that the nascent polypeptide from the translocon threads through this groove while being scanned by the Ost1p subunit for the presence of the glycosylation sequon.

Dimeric organization of the yeast oligosaccharyl transferase complex

The enzyme complex oligosaccharyl transferase (OT) catalyzes N-glycosylation in the lumen of the endoplasmic reticulum. The yeast OT complex is composed of nine subunits, all of which are transmembrane proteins. Several lines of evidence, including our previous split-ubiquitin studies, have suggested an oligomeric organization of the OT complex, but the exact oligomeric nature has been unclear. By FLAG epitope tagging the Ost4p subunit of the OT complex, we purified the OT enzyme complex by using the nondenaturing detergent digitonin and a one-step immunoaffinity technique. The digitonin-solubilized OT complex was catalytically active, and all nine subunits were present in the enzyme complex. The purified OT complex had an apparent mass of approximately 500 kDa, suggesting a dimeric configuration, which was confirmed by biochemical studies. EM showed homogenous individual particles and revealed a dimeric structure of the OT complexes that was consistent with our biochemical studies. A 3D structure of the dimeric OT complex at 25-A resolution was reconstructed from EM images. We suggest that the dimeric structure of OT might be required for effective association with the translocon dimer and for its allosteric regulation during cotranslational glycosylation.

Asparagine-linked protein glycosylation: from eukaryotic to prokaryotic systems

Asparagine-linked protein glycosylation is a prevalent protein modification reaction in eukaryotic systems. This process involves the co-translational transfer of a pre-assembled tetradecasaccharide from a dolichyl-pyrophosphate donor to the asparagine side chain of nascent proteins at the endoplasmic reticulum (ER) membrane. Recently, the first such system of N-linked glycosylation was discovered in the Gram-negative bacterium, Campylobacter jejuni. Glycosylation in this organism involves the transfer of a heptasaccharide from an undecaprenyl-pyrophosphate donor to the asparagine side chain of proteins at the bacterial periplasmic membrane. Here we provide a detailed comparison of the machinery involved in the N-linked glycosylation systems of eukaryotic organisms, exemplified by the yeast Saccharomyces cerevisiae, with that of the bacterial system in C. jejuni. The two systems display significant similarities and the relative simplicity of the bacterial glycosylation process could provide a model system that can be used to decipher the complex eukaryotic glycosylation machinery.

Subunits of the translocon interact with components of the oligosaccharyl transferase complex

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.