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Crystal structures of an archaeal oligosaccharyltransferase provide insights into the catalytic cycle of N-linked protein glycosylation.

ABSTRACT: Oligosaccharyltransferase transfers an oligosaccharide chain to the asparagine residues in proteins. The archaeal and eubacterial oligosaccharyltransferases are single subunit membrane enzymes, referred to as "AglB" (archaeal glycosylation B) and "PglB" (protein glycosylation B), respectively. Only one crystal structure of a full-length PglB has been solved. Here we report the crystal structures of the full-length AglB from a hyperthermophilic archaeon, Archaeoglobus fulgidus. The AglB and PglB proteins share the common overall topology of the 13 transmembrane helices, and a characteristic long plastic loop in the transmembrane region. This is the structural basis for the formation of the catalytic center, consisting of conserved acidic residues coordinating a divalent metal ion. In one crystal form, a sulfate ion was bound next to the metal ion. This structure appears to represent a dolichol-phosphate binding state, and suggests the release mechanism for the glycosylated product. The structure in the other crystal form corresponds to the resting state conformation with the well-ordered plastic loop in the transmembrane region. The overall structural similarity between the distantly related AglB and PglB proteins strongly indicates the conserved catalytic mechanism in the eukaryotic counterpart, the STT3 (stauroporine and temperature sensitivity 3) protein. The detailed structural comparison provided the dynamic view of the N-glycosylation reaction, involving the conversion between the structured and unstructured states of the plastic loop in the transmembrane region and the formation and collapse of the Ser/Thr-binding pocket in the C-terminal globular domain.

SUBMITTER: Matsumoto S

PROVIDER: S-EPMC3816453 | biostudies-literature | 2013 Oct

REPOSITORIES: biostudies-literature

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Crystal structures of an archaeal oligosaccharyltransferase provide insights into the catalytic cycle of N-linked protein glycosylation.

Matsumoto Shunsuke S Shimada Atsushi A Nyirenda James J Igura Mayumi M Kawano Yoshiaki Y Kohda Daisuke D

Proceedings of the National Academy of Sciences of the United States of America 20131014 44

Oligosaccharyltransferase transfers an oligosaccharide chain to the asparagine residues in proteins. The archaeal and eubacterial oligosaccharyltransferases are single subunit membrane enzymes, referred to as "AglB" (archaeal glycosylation B) and "PglB" (protein glycosylation B), respectively. Only one crystal structure of a full-length PglB has been solved. Here we report the crystal structures of the full-length AglB from a hyperthermophilic archaeon, Archaeoglobus fulgidus. The AglB and PglB ...[more]

PMID: 24127570

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Project description:Asparagine-linked glycosylation, also known as N-linked glycosylation is an essential and highly conserved post-translational protein modification that occurs in all three domains of life. This modification is essential for specific molecular recognition, protein folding, sorting in the endoplasmic reticulum, cell-cell communication, and stability. Defects in N-linked glycosylation results in a class of inherited diseases known as congenital disorders of glycosylation (CDG). N-linked glycosylation occurs in the endoplasmic reticulum (ER) lumen by a membrane associated enzyme complex called the oligosaccharyltransferase (OST). In the central step of this reaction, an oligosaccharide group is transferred from a lipid-linked dolichol pyrophosphate donor to the acceptor substrate, the side chain of a specific asparagine residue of a newly synthesized protein. The prokaryotic OST enzyme consists of a single polypeptide chain, also known as single subunit OST or ssOST. In contrast, the eukaryotic OST is a complex of multiple non-identical subunits. In this review, we will discuss the biochemical and structural characterization of the prokaryotic, yeast, and mammalian OST enzymes. This review explains the most recent high-resolution structures of OST determined thus far and the mechanistic implication of N-linked glycosylation throughout all domains of life. It has been shown that the ssOST enzyme, AglB protein of the archaeon Archaeoglobus fulgidus, and the PglB protein of the bacterium Campylobactor lari are structurally and functionally similar to the catalytic Stt3 subunit of the eukaryotic OST enzyme complex. Yeast OST enzyme complex contains a single Stt3 subunit, whereas the human OST complex is formed with either STT3A or STT3B, two paralogues of Stt3. Both human OST complexes, OST-A (with STT3A) and OST-B (containing STT3B), are involved in the N-linked glycosylation of proteins in the ER. The cryo-EM structures of both human OST-A and OST-B complexes were reported recently. An acceptor peptide and a donor substrate (dolichylphosphate) were observed to be bound to the OST-B complex whereas only dolichylphosphate was bound to the OST-A complex suggesting disparate affinities of two OST complexes for the acceptor substrates. However, we still lack an understanding of the independent role of each eukaryotic OST subunit in N-linked glycosylation or in the stabilization of the enzyme complex. Discerning the role of each subunit through structure and function studies will potentially reveal the mechanistic details of N-linked glycosylation in higher organisms. Thus, getting an insight into the requirement of multiple non-identical subunits in the N-linked glycosylation process in eukaryotes poses an important future goal.

Project description:BackgroundAsparagine (N)-linked glycosylation is one of the most crucial post-translational modifications, which is catalyzed in the lumen of the endoplasmic reticulum (ER) by the oligosaccharyltransferase (OST) in eukaryotic cells. Biochemical and genetic assay leads to the identification of the nine subunits (Ost 1-6, Stt3, Swp1 and Wbp1) of the yeast OST and in which Stt3p is proposed playing a central and conserved role in N-glycosylation. Two STT3 isoform genes, STT3A and STT3B, exist in the plant and mammal genomes. OST with different catalytic STT3 isoforms has different enzymatic properties in mammals. The mutation of STT3A in Arabidopsis thaliana causes a salt hypersensitive phenotype the inhibited root growth and swollen root tips suggesting protein N-glycosylation is indispensable for plant growth and development. Spartina alterniflora is widely used for shoreline protection and tidal marsh restoration due to the strong salt tolerance although the exact molecular mechanism is little known. To explore the possible biological roles of N-glycosylation in plant adaptive resistance to salinity stress, we cloned the STT3 genes from S. alterniflora and heterogenously expressed them in Arabidopsis mutant to observe the functional conservation.ResultsSaSTT3A and SaSTT3B genes were cloned from Spartina alterniflora. SaSTT3A genomic sequences spanned over 23 exons and 22 introns, while SaSTT3B had 6 exons and 5 introns. The gene structures of both genes were conserved among the analyzed plant species. Subcellular localization and transmembrane structure prediction revealed that these two genes had 13 and 11 transmembrane helices respectively. The functional complementation in which the cDNA of SaSTT3A and SaSTT3B driven by CaMV 35S promoter completely or partially rescued Arabidopsis stt3a-2 mutant salt-sensitive phenotype, indicating STT3A functions conservatively between glycophyte and halophyte and N-glycosylation might be involved in plant resistance to salinity.ConclusionsTwo STT3 isoform genes, SaSTT3A and SaSTT3B, were cloned from S. alterniflora and they were evolutionally conserved at gene structure and coding sequences compared with their counterparts. Moreover, SaSTT3 genes could successfully rescue Arabidopsis stt3a-2 salt-sensitive phenotype, suggesting there exists a similar N-glycosylation process in S. alterniflora. Here we provided a first piece of evidence that the N-glycosylation might be involved in salt tolerance of halophyte.

Dataset Information

Crystal structures of an archaeal oligosaccharyltransferase provide insights into the catalytic cycle of N-linked protein glycosylation.

Publications

Crystal structures of an archaeal oligosaccharyltransferase provide insights into the catalytic cycle of N-linked protein glycosylation.

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