Vitamin B 12 (cobalamin, Cbl) is essential to the function of two human enzymes, methionine synthase (MS) and methylmalonyl-CoA mutase (MUT). The conversion of dietary Cbl to its cofactor forms, methyl-Cbl (MeCbl) for MS and adenosyl-Cbl (AdoCbl) for MUT, located in the cytosol and mitochondria, respectively, requires a complex pathway of intracellular processing and trafficking. One of the processing proteins, MMAA (methylmalonic aciduria type A), is implicated in the mitochondrial assembly of AdoCbl into MUT and is defective in children from the cblA complementation group of cobalamin disorders. To characterize the functional interplay between MMAA and MUT, we have crystallized human MMAA in the GDP-bound form and human MUT in the apo, holo, and substrate-bound ternary forms. Structures of both proteins reveal highly conserved domain architecture and catalytic machinery for ligand binding, yet they show substantially different dimeric assembly and interaction, compared with their bacterial counterparts. We show that MMAA exhibits GTPase activity that is modulated by MUT and that the two proteins interact in vitro and in vivo. Formation of a stable MMAA-MUT complex is nucleotideselective for MMAA (GMPPNP over GDP) and apoenzyme-dependent for MUT. The physiological importance of this interaction is highlighted by a recently identified homoallelic patient mutation of MMAA, G188R, which, we show, retains basal GTPase activity but has abrogated interaction. Together, our data point to a gatekeeping role for MMAA by favoring complex formation with MUT apoenzyme for AdoCbl assembly and releasing the AdoCbl-loaded holoenzyme from the complex, in a GTP-dependent manner.

The innate immune system is critical in the response to infection by pathogens and it is activated by pattern recognition receptors (PRRs) binding to pathogen associated molecular patterns (PAMPs). During viral infection, the direct recognition of the viral nucleic acids, such as the genomes of DNA viruses, is very important for activation of innate immunity. Recently, DNA-dependent protein kinase (DNA-PK), a heterotrimeric complex consisting of the Ku70/Ku80 heterodimer and the catalytic subunit DNA-PKcs was identified as a cytoplasmic PRR for DNA that is important for the innate immune response to intracellular DNA and DNA virus infection. Here we show that vaccinia virus (VACV) has evolved to inhibit this function of DNA-PK by expression of a highly conserved protein called C16, which was known to contribute to virulence but by an unknown mechanism. Data presented show that C16 binds directly to the Ku heterodimer and thereby inhibits the innate immune response to DNA in fibroblasts, characterised by the decreased production of cytokines and chemokines. Mechanistically, C16 acts by blocking DNA-PK binding to DNA, which correlates with reduced DNA-PK-dependent DNA sensing. The C-terminal region of C16 is sufficient for binding Ku and this activity is conserved in the variola virus (VARV) orthologue of C16. In contrast, deletion of 5 amino acids in this domain is enough to knockout this function from the attenuated vaccine strain modified vaccinia virus Ankara (MVA). In vivo a VACV mutant lacking C16 induced higher levels of cytokines and chemokines early after infection compared to control viruses, confirming the role of this virulence factor in attenuating the innate immune response. Overall this study describes the inhibition of DNA-PK-dependent DNA sensing by a poxvirus protein, adding to the evidence that DNA-PK is a critical component of innate immunity to DNA viruses.

Viruses have evolved sophisticated strategies to exploit host cell function for their benefit. Here we show that under physiologically normal oxygen levels (normoxia) vaccinia virus (VACV) infection leads to a rapid stabilization of hypoxia-inducible factor (HIF)-1α, its translocation into the nucleus and the activation of HIF-responsive genes, such as vascular endothelial growth factor (VEGF), glucose transporter-1, and pyruvate dehydrogenase kinase-1. HIF-1α stabilization is mediated by VACV protein C16 that binds the human oxygen sensing enzyme prolyl-hydroxylase domain containing protein (PHD)2 and thereby inhibits PHD2-dependent hydroxylation of HIF-1α. The binding between C16 and PHD2 is direct and specific, and ectopic expression of C16 alone induces transcription of HIF-1α responsive genes. Conversely, a VACV strain lacking the gene for C16, C16L, is unable to induce HIF-1α stabilization. Interestingly, the N-terminal region of C16 is predicted to have a PHD2-like structural fold but lacks the catalytic active site residues of PHDs. The induction of a hypoxic response by VACV is reminiscent of the biochemical consequences of solid tumor formation, and illustrates a poxvirus strategy for manipulation of cellular gene expression and biochemistry.Orthopoxvirus | variola virus | Warburg effect | glucose metabolism V accinia virus (VACV) is the live vaccine used against smallpox, an extinct human disease caused by variola virus (VARV) (1). VACV and VARV belong to the Orthopoxvirus genus of the Poxviridae, and each have large dsDNA genomes and replicate in the cell cytoplasm (2). VACV can be engineered to express foreign genes and these recombinant viruses have potential as live vaccines against other diseases (3) and are useful laboratory tools, for instance in identifying the target antigens of CD8+ cytotoxic T lymphocytes (4, 5). VACV encodes many genes that are nonessential for virus replication but are important in vivo for defense against the host innate immune system, such as the VACV steroid biosynthetic enzyme 3-β-hydroxysteroid dehydrogenase (6, 7), or for modulation of host cell biochemistry, such as the VACV enzymes thymidine kinase (8), thymidylate kinase (9), and ribonuceotide reductase (10).VACV strain Western Reserve (WR) gene C16L encodes a 37-kDa intracellular protein that contributes to virulence by an unknown mechanism (11). C16 is nonessential for VACV replication, yet orthologs of C16 are conserved in several poxviruses, suggesting an important function (11). VACV also encodes a related protein, C4, with 43% amino acid identity to C16 and that inhibits activation of NF-κB (12).Here a direct interaction is shown between the N-terminal region of C16 and the human oxygen sensor prolyl-hydroxylase domain containing protein (PHD)2. During normoxia, PHD2 is the major enzyme that hydroxylates hypoxia-inducible factor (HIF)-1α on either of two proline residues within the oxygendependent degradation domain (ODD), P402 and P564 (13, 14). After hydroxylation, HIF-1α is ubiquitinated by the von Hippe...

BackgroundThe BAG6 protein is a subunit of a heterotrimeric complex that binds a range of membrane and secretory protein precursors localized to the cytosol, enforcing quality control and influencing their subsequent fate.Methodology and Principal FindingsBAG6 has an N-terminal ubiquitin-like domain, and a C-terminal Bcl-2-associated athanogene domain, separated by a large central proline-rich region. We have used in vitro binding approaches to identify regions of BAG6 important for its interactions with: i) the small-glutamine rich tetratricopeptide repeat-containing protein alpha (SGTA) and ii) two model tail-anchored membrane proteins as a paradigm for its hydrophobic substrates. We show that the BAG6-UBL is essential for binding to SGTA, and find that the UBL of a second subunit of the BAG6-complex, ubiquitin-like protein 4A (UBL4A), competes for SGTA binding. Our data show that this binding is selective, and suggest that SGTA can bind either BAG6, or UBL4A, but not both at the same time. We adapted our in vitro binding assay to study the association of BAG6 with an immobilized tail-anchored protein, Sec61β, and find both the UBL and BAG domains are dispensable for binding this substrate. This conclusion was further supported using a heterologous subcellular localization assay in yeast, where the BAG6-dependent nuclear relocalization of a second tail-anchored protein, GFP-Sed5, also required neither the UBL, nor the BAG domain of BAG6.SignificanceOn the basis of these findings, we propose a working model where the large central region of the BAG6 protein provides a binding site for a diverse group of substrates, many of which expose a hydrophobic stretch of polypeptide. This arrangement would enable the BAG6 complex to bring together its substrates with potential effectors including those recruited via its N-terminal UBL. Such effectors may include SGTA, and the resulting assemblies influence the subsequent fate of the hydrophobic BAG6 substrates.