Analysis of the costructure of the simian virus 40 T-antigen origin binding domain with site I reveals a correlation between GAGGC spacing and spiral assembly

A Syntenic Pangenome for Gardnerella Reveals Taxonomic Boundaries and Stratification of Metabolic and Virulence Potential across Species

Bacterial vaginosis (BV) is a prevalent condition associated with an imbalance in the vaginal microbiota, often involving species of Gardnerella . The taxonomic complexity and inconsistent nomenclature of Gardnerella have impeded progress in understanding the role of specific species in health and disease. In this study, we conducted a comprehensive genomic and pangenomic analysis to resolve taxonomic ambiguities and elucidate metabolic and virulence potential across Gardnerella species. We obtained complete, closed genomes for 42 Gardnerella isolates from women with BV and curated publicly available genome sequences (n = 291). Average nucleotide identity (ANI) analysis, digital DNA-DNA hybridization (dDDH), and the cpn60 gene sequences identified nine species and eleven subspecies within Gardnerella , for which we refined species and subspecies boundaries and proposed updated nomenclature. Pangenome analysis revealed species-specific gene clusters linked to metabolic pathways, virulence factors, and niche adaptations, distinguishing species specialized for mucin degradation in the vaginal environment from those potentially adapted to urinary tract colonization. Notably, we identified lineage-specific evolutionary divergence in gene clusters associated with biofilm formation, carbohydrate metabolism, and antimicrobial resistance. We further discovered the first cryptic plasmids naturally present within the Gardnerella genus. Our findings provide a unified framework for Gardnerella taxonomy and nomenclature, and enhance our understanding of species-specific functional capabilities, with implications for Gardnerella research, diagnostics, and targeted therapeutics in BV.

Polyomavirus large T antigens: Unraveling a complex interactome

All members of the polyomavirus family encode a large T antigen (LT) protein that plays essential roles in viral DNA replication, regulation of viral gene expression, and the manipulation of numerous cellular pathways. Over 100 polyomaviruses have been discovered in hosts ranging from arthropods and fish to mammals, including fourteen that infect humans. LT is among the most studied viral proteins with thousands of articles describing its functions in viral productive infection and tumorigenesis. However, nearly all knowledge of LT activities is based on the studies of simian virus 40 (SV40) and a few other viruses. Comparative studies of LT proteins of different polyomaviruses have revealed a remarkable diversity in the mechanisms by which LT proteins function across different polyomavirus species. This review focuses on human polyomaviruses highlights the similarities and differences between polyomavirus LTs and highlights gaps in our understanding of this protein family. The concentration of knowledge around SV40 LT and the corresponding lack of mechanistic studies on LT proteins encoded by other human and animal polyomaviruses severely constrains our understanding of the biology of this important virus family.

A model for polyomavirus helicase activity derived in part from the AlphaFold2 structure of SV40 T-antigen

The mechanism used by polyomavirus and other viral SF3 helicases to unwind DNA at replication forks remains unknown. Using AlphaFold2, we have determined the structure of a representative SF3 helicase, the SV40 T-antigen (T-ag). This model has been analyzed in terms of the features of T-ag required for helicase activity, particularly the proximity of the T-ag origin binding domain (OBD) to the replication fork and the distribution of basic residues on the surface of the OBD that are known to play roles in DNA unwinding. These and related studies provide additional evidence that the T-ag OBDs have a role in the unwinding of DNA at the replication fork. Nuclear magnetic resonance and modeling experiments also indicate that protonated histidines on the surface of the T-ag OBD play an important role in the unwinding process, and additional modeling studies indicate that protonated histidines are essential in other SF3 and SF6 helicases. Finally, a model for T-ag's helicase activity is presented, which is a variant of the "rope climber." According to this model, the hands are the N-terminal OBD domains that interact with the replication fork, while the C-terminal helicase domains contain the feet that bind to single-stranded DNA.

IMPORTANCE

Enzymes termed helicases are essential for the replication of DNA tumor viruses. Unfortunately, much remains to be determined about this class of enzymes, including their structures and the mechanism(s) they employ to unwind DNA. Herein, we present the full-length structure of a model helicase encoded by a DNA tumor virus. Moreover, this AI-based structure has been analyzed in terms of its basic functional properties, such as the orientation of the helicase at replication forks and the relative locations of the amino acid residues that are critical for helicase activity. Obtaining this information is important because it permits proposals regarding how DNA is routed through these model helicases. Also presented is structural evidence that the conclusions drawn from our detailed analyses of one model helicase, encoded by one class of tumor viruses, are likely to apply to other viral and eukaryotic helicases.

Crystal structure of an engineered, HIV-specific recombinase for removal of integrated proviral DNA

As part of the HIV infection cycle, viral DNA inserts into the genome of host cells such that the integrated DNA encoding the viral proteins is flanked by long terminal repeat (LTR) regions from the retrovirus. In an effort to develop novel genome editing techniques that safely excise HIV provirus from cells, Tre, an engineered version of Cre recombinase, was designed to target a 34-bp sequence within the HIV-1 LTR (loxLTR). The sequence targeted by Tre lacks the symmetry present in loxP, the natural DNA substrate for Cre. We report here the crystal structure of a catalytically inactive (Y324F) mutant of this engineered Tre recombinase in complex with the loxLTR DNA substrate. We also report that 17 of the 19 amino acid changes relative to Cre contribute to the altered specificity, even though many of these residues do not contact the DNA directly. We hypothesize that some mutations increase the flexibility of the Cre tetramer and that this, along with flexibility in the DNA, enable the engineered enzyme and DNA substrate to adopt complementary conformations.

The structure of SV40 large T hexameric helicase in complex with AT-rich origin DNA

DNA replication is a fundamental biological process. The initial step in eukaryotic DNA replication is the assembly of the pre-initiation complex, including the formation of two head-to-head hexameric helicases around the replication origin. How these hexameric helicases interact with their origin dsDNA remains unknown. Here, we report the co-crystal structure of the SV40 Large-T Antigen (LT) hexameric helicase bound to its origin dsDNA. The structure shows that the six subunits form a near-planar ring that interacts with the origin, so that each subunit makes unique contacts with the DNA. The origin dsDNA inside the narrower AAA+ domain channel shows partial melting due to the compression of the two phosphate backbones, forcing Watson-Crick base-pairs within the duplex to flip outward. This structure provides the first snapshot of a hexameric helicase binding to origin dsDNA, and suggests a possible mechanism of origin melting by LT during SV40 replication in eukaryotic cells.

Structural Based Analyses of the JC Virus T-Antigen F258L Mutant Provides Evidence for DNA Dependent Conformational Changes in the C-Termini of Polyomavirus Origin Binding Domains

The replication of human polyomavirus JCV, which causes Progressive Multifocal Leukoencephalopathy, is initiated by the virally encoded T-antigen (T-ag). The structure of the JC virus T-ag origin-binding domain (OBD) was recently solved by X-ray crystallography. This structure revealed that the OBD contains a C-terminal pocket, and that residues from the multifunctional A1 and B2 motifs situated on a neighboring OBD molecule dock into the pocket. Related studies established that a mutation in a pocket residue (F258L) rendered JCV T-ag unable to support JCV DNA replication. To establish why this mutation inactivated JCV T-ag, we have solved the structure of the F258L JCV T-ag OBD mutant. Based on this structure, it is concluded that the structural consequences of the F258L mutation are limited to the pocket region. Further analyses, utilizing the available polyomavirus OBD structures, indicate that the F258 region is highly dynamic and that the relative positions of F258 are governed by DNA binding. The possible functional consequences of the DNA dependent rearrangements, including promotion of OBD cycling at the replication fork, are discussed.

Insights into the initiation of JC virus DNA replication derived from the crystal structure of the T-antigen origin binding domain

JC virus is a member of the Polyomavirus family of DNA tumor viruses and the causative agent of progressive multifocal leukoencephalopathy (PML). PML is a disease that occurs primarily in people who are immunocompromised and is usually fatal. As with other Polyomavirus family members, the replication of JC virus (JCV) DNA is dependent upon the virally encoded protein T-antigen. To further our understanding of JCV replication, we have determined the crystal structure of the origin-binding domain (OBD) of JCV T-antigen. This structure provides the first molecular understanding of JCV T-ag replication functions; for example, it suggests how the JCV T-ag OBD site-specifically binds to the major groove of GAGGC sequences in the origin. Furthermore, these studies suggest how the JCV OBDs interact during subsequent oligomerization events. We also report that the OBD contains a novel "pocket"; which sequesters the A1 & B2 loops of neighboring molecules. Mutagenesis of a residue in the pocket associated with the JCV T-ag OBD interfered with viral replication. Finally, we report that relative to the SV40 OBD, the surface of the JCV OBD contains one hemisphere that is highly conserved and one that is highly variable.