Adam S. Olia

Three-dimensional structure of a viral genome-delivery portal vertex

DNA viruses such as bacteriophages and herpesviruses deliver their genome into and out of the capsid through large proteinaceous assemblies, known as portal proteins. Here, we report two snapshots of the dodecameric portal protein of bacteriophage P22. The 3.25-Å-resolution structure of the portal-protein core bound to 12 copies of gene product 4 (gp4) reveals a ~1.1-MDa assembly formed by 24 proteins. Unexpectedly, a lower-resolution structure of the full-length portal protein unveils the unique topology of the C-terminal domain, which forms a ~200-Å-long α-helical barrel. This domain inserts deeply into the virion and is highly conserved in the Podoviridae family. We propose that the barrel domain facilitates genome spooling onto the interior surface of the capsid during genome packaging and, in analogy to a rifle barrel, increases the accuracy of genome ejection into the host cell.

Determination of Stoichiometry and Conformational Changes in the First Step of the P22 Tail Assembly

Large oligomeric portal assemblies have a central role in the life-cycles of bacteriophages and herpesviruses. The stoichiometry of in vitro assembled portal proteins has been a subject of debate for several years. The intrinsic polymorphic oligomerization of ectopically expressed portal proteins makes it possible to form rings of diverse stoichiometry (e.g., 11-mer, 12-mer, 13-mer, etc.) in solution. In this study, we have investigated the stoichiometry of the in vitro-assembled portal protein of bacteriophage P22 and characterized its association with the tail factor gp4. Using native mass spectrometry, we show for the first time that the reconstituted portal protein (assembled in vitro using a modified purification and assembly protocol) is exclusively dodecameric. Under the conditions used here, 12 copies of tail factor gp4 bind to the portal ring, in a cooperative fashion, to form a 12:12 complex of 1.050 MDa. We applied tandem mass spectrometry to the complete assembly and found an unusual dimeric dissociation pattern of gp4, suggesting a dimeric sub-organization of gp4 when assembled with the portal ring. Furthermore, native and ion mobility mass spectrometry reveal a major conformational change in the portal upon binding of gp4. We propose that the gp4-induced conformational change in the portal ring initiates a cascade of events assisting in the stabilization of newly filled P22 particles, which marks the end of phage morphogenesis.

Molecular basis for the recognition of snurportin 1 by importin beta.

The nuclear import of uridine-rich ribonucleoproteins is mediated by the transport adaptor snurportin 1 (SNP1). Similar to importin α, SNP1 uses an N-terminal importin β binding (sIBB) domain to recruit the receptor importin β and gain access to the nucleus. In this study, we demonstrate that the sIBB domain has a bipartite nature, which contains two distinct binding determinants for importin β. The first determinant spans residues 25-65 and includes the previously identified importin α IBB (αIBB) region of homology. The second binding determinant encompasses residues 1-24 and resembles region 1011-1035 of the nucleoporin 153 (Nup153). The two binding determinants synergize within the sIBB domain to confer a low nanomolar binding affinity for importin β (Kd ∼ 2 nm) in an interaction that, in vitro, is displaced by RanGTP. We propose that in vivo the synergy of Nup153 and nuclear RanGTP promotes translocation of uridine-rich ribonucleoproteins into the nucleus.

Structure of phage P22 cell envelope–penetrating needle.

Bacteriophage P22 infects Salmonella enterica by injecting its genetic material through the cell envelope. During infection, a specialized tail needle, gp26, is injected into the host, likely piercing a hole in the host cell envelope. The 2.1-Å crystal structure of gp26 reveals a 240-Å elongated protein fiber formed by two trimeric coiled-coil domains interrupted by a triple β-helix. The N terminus of gp26 plugs the portal protein channel, retaining the genetic material inside the virion. The C-terminal tip of the fiber exposes β-hairpins with hydrophobic tips similar to those seen in class II fusion peptides. The α-helical core connecting these two functionally polarized tips presents four trimerization octads with consensus sequence IXXLXXXV. The slender conformation of the gp26 fiber minimizes the surface exposed to solvent, which is consistent with the idea that gp26 traverses the cell envelope lipid bilayers.

Nonenzymatic Protein Acetylation Detected by NAPPA Protein Arrays

Acetylation is a post-translational modification that occurs on thousands of proteins located in many cellular organelles. This process mediates many protein functions and modulates diverse biological processes. In mammalian cells, where acetyl-CoA is the primary acetyl donor, acetylation in the mitochondria is thought to occur by chemical means due to the relatively high concentration of acetyl-CoA located in this organelle. In contrast, acetylation outside of the mitochondria is thought to be mediated predominantly by acetyltransferase enzymes. Here, we address the possibility that nonenzymatic chemical acetylation outside of the mitochondria may be more common than previously appreciated. We employed the Nucleic Acid Programmable Protein Array platform to perform an unbiased screen for human proteins that undergo chemical acetylation, which resulted in the identification of a multitude of proteins with diverse functions and cellular localization. Mass spectrometry analysis revealed that basic residues typically precede the acetylated lysine in the -7 to -3 position, and we show by mutagenesis that these basic residues contribute to chemical acetylation capacity. We propose that these basic residues lower the pKa of the substrate lysine for efficient chemical acetylation. Many of the identified proteins reside outside of the mitochondria and have been previously demonstrated to be acetylated in vivo. As such, our studies demonstrate that chemical acetylation occurs more broadly throughout the eukaryotic cell than previously appreciated and suggests that this post-translational protein modification may have more diverse roles in protein function and pathway regulation.

Architecture of viral genome-delivery molecular machines

From the abyss of the ocean to the human gut, bacterial viruses (or bacteriophages) have colonized all ecosystems of the planet earth and evolved in sync with their bacterial hosts. Over 95% of bacteriophages have a tail that varies greatly in length and complexity. The tail complex interrupts the icosahedral capsid symmetry and provides both an entry for viral genome-packaging during replication and an exit for genome-ejection during infection. Here, we review recent progress in deciphering the structure, assembly and conformational dynamics of viral genome-delivery tail machines. We focus on the bacteriophages P22 and T7, two well-studied members of the Podoviridae family that use short, non-contractile tails to infect Gram-negative bacteria. The structure of specialized tail fibers and their putative role in host anchoring, cell-surface penetration and genome-ejection is discussed.

Structural plasticity of the phage P22 tail needle gp26 probed with xenon gas.

The tail needle, gp26, is a highly stable homo‐trimeric fiber found in the tail apparatus of bacteriophage P22. In the mature virion, gp26 is responsible for plugging the DNA exit channel, and likely plays an important role in penetrating the host cell envelope. In this article, we have determined the 1.98 Å resolution crystal structure of gp26 bound to xenon gas. The structure led us to identify a calcium and a chloride ion intimately bound at the interior of α‐helical core, as well as seven small cavities occupied by xenon atoms. The two ions engage in buried polar interactions with gp26 side chains that provide specificity and register to gp26 helical core, thus enhancing its stability. Conversely, the distribution of xenon accessible cavities correlates well with the flexibility of the fiber observed in solution and in the crystal structure. We suggest that small internal cavities in gp26 between the helical core and the C‐terminal tip allow for flexible swinging of the latter, without affecting the overall stability of the protein. The C‐terminal tip may be important in scanning the bacterial surface in search of a cell‐envelope penetration site, or for recognition of a yet unidentified receptor on the surface of the host.

Structural Plasticity of the Protein Plug That Traps Newly Packaged Genomes in Podoviridae Virions

Bacterial viruses of the P22-like family encode a specialized tail needle essential for genome stabilization after DNA packaging and implicated in Gram-negative cell envelope penetration. The atomic structure of P22 tail needle (gp26) crystallized at acidic pH reveals a slender fiber containing an N-terminal “trimer of hairpins” tip. Although the length and composition of tail needles vary significantly in Podoviridae, unexpectedly, the amino acid sequence of the N-terminal tip is exceptionally conserved in more than 200 genomes of P22-like phages and prophages. In this paper, we used x-ray crystallography and EM to investigate the neutral pH structure of three tail needles from bacteriophage P22, HK620, and Sf6. In all cases, we found that the N-terminal tip is poorly structured, in stark contrast to the compact trimer of hairpins seen in gp26 crystallized at acidic pH. Hydrogen-deuterium exchange mass spectrometry, limited proteolysis, circular dichroism spectroscopy, and gel filtration chromatography revealed that the N-terminal tip is highly dynamic in solution and unlikely to adopt a stable trimeric conformation at physiological pH. This is supported by the cryo-EM reconstruction of P22 mature virion tail, where the density of gp26 N-terminal tip is incompatible with a trimer of hairpins. We propose the tail needle N-terminal tip exists in two conformations: a pre-ejection extended conformation, which seals the portal vertex after genome packaging, and a postejection trimer of hairpins, which forms upon its release from the virion. The conformational plasticity of the tail needle N-terminal tip is built in the amino acid sequence, explaining its extraordinary conservation in nature.

Molecular Basis for Cohesin Acetylation by Establishment of Sister Chromatid Cohesion N-acetyltransferase ESCO1

Protein acetylation is a prevalent posttranslational modification that is regulated by diverse acetyltransferase enzymes. Although histone acetyltransferases (HATs) have been well characterized both structurally and mechanistically, far less is known about non-histone acetyltransferase enzymes. The human ESCO1 and ESCO2 paralogs acetylate the cohesin complex subunit SMC3 to regulate the separation of sister chromatids during mitosis and meiosis. Missense mutations within the acetyltransferase domain of these proteins correlate with diseases, including endometrial cancers and Roberts syndrome. Despite their biological importance, the mechanisms underlying acetylation by the ESCO proteins are not understood. Here, we report the X-ray crystal structure of the highly conserved zinc finger-acetyltransferase moiety of ESCO1 with accompanying structure-based mutagenesis and biochemical characterization. We find that the ESCO1 acetyltransferase core is structurally homologous to the Gcn5 HAT, but contains unique additional features including a zinc finger and an ∼40-residue loop region that appear to play roles in protein stability and SMC3 substrate binding. We identify key residues that play roles in substrate binding and catalysis, and rationalize the functional consequences of disease-associated mutations. Together, these studies reveal the molecular basis for SMC3 acetylation by ESCO1 and have broader implications for understanding the structure/function of non-histone acetyltransferases.

A shifty stop for a hairy tail.

The tail apparatus of the bacteriophage SPP1 is an extraordinary ∼1600‐Å‐long molecular machine. The tail mediates attachment of the virus to the host surface receptor, as well‐as ejection of the viral genome into the host. The distal tip of the tail binds the extracellular ectodomain of the Bacillus subtilis receptor YueB, while the tail tube provides a conduit to funnel the viral genome into the host. This process, which culminates with the ejection of the ∼44 kb of viral DNA across the thick, cell envelope of the Gram‐positive bacterial cell, takes place in a time scale of seconds to minutes and represents a remarkable example of biotransformation. In this issue of Molecular Microbiology, Auzat et al. provide compelling evidence that the two major structural proteins of the SPP1 tail, gp17.1 (∼19.1 kDa) and gp17.1* (∼28 kDa), share a common N‐terminal sequence, and that gp17.1* is generated by a translational frameshift in the gene 17.1. The extra domain fused to gp17.1* is synthesized by a +1 programmed translational frameshift at the end of gene 17.1, which leads to the synthesis of approximately one gp17.1* for every three equivalents of gp17.1. This finding extends our current knowledge of translational frameshifts and provides a framework to understand how Siphoviridae phages like SPP1 have developed long‐tail machines using only two major structural proteins.