RESEARCH HIGHLIGHTS VIRUS STRUCTURE The structure of a virus that infects a have been created for these archaeal hyperthermophilic archaeon has just viruses: Fuselloviridae (spindle- been published in the Proceedings of shaped), Rudiviridae (rod-shaped), the National Academy of Sciences Lipothrixviridae (filamentous) and Founding member (USA). The major coat protein in the Guttaviridae (teardrop). capsid has striking similarities to The virus isolated in this study both animal and bacterial viruses — replicates in a close relative of raising the exciting prospect that Sulfolobus solfataricus, but turned out some viruses might have a common to be different to the other viruses pre- ancestor from more than 3 billion viously isolated from Sulfolobus species. years ago. The double-stranded DNA genome Of more than 5,000 viruses that contained only 3 open reading frames have been characterized, only a tiny (ORFs) that had any similarity to fraction are parasites of archaeal previously sequenced genes. Using species. Hyperthermophilic archaea cryoTEM (transmission electron inhabit hostile niches such as boil- microscopy) to solve the structure of ing hot springs, and evolutionary the virus particle to 27-Å resolution, analyses indicate that this group is Rice and co-workers found surprising one of the most ancient lineages in structural features. The capsid is the tree of life. Virus evolution has icosahedral with unusual turret-like not been as easy to address, owing to protruberances, so the authors have the lack of homology between viral named this virus STIV — Sulfolobus genes. Now, attention has focused turreted icosahedral virus. on structural similarities between Importantly, the STIV capsid has a viral proteins. similar organization to the capsids of Viruses that replicate in archaeal the human adenovirus, bacteriophage species have been isolated in the PRD1 and an algal virus PBCV-1. past, including four viruses that Moreover, docking the crystal struc- replicate in Sulfolobus species, but tures of the major capsid proteins each virus is unrelated to other of human adenovirus and PRD1 CryoTEM and image reconstruction of STIV. Turrets extend 13 nm above the virus surface. viruses using genome sequence onto the STIV major capsid protein Image courtesy of Mark Young, (Montana State University-Bozeman, USA). comparisons. Four new families revealed that all three proteins have a BACTERIAL PHYSIOLOGY revealed that the streptococci contain only the Again, using immunogold electron general secretory (Sec) pathway for protein microscopy, the authors were able to show export across the cytoplasmic membrane. that secreted proteins and the Sec What remained unclear, however, was an translocation machinery co-localized to the Positive secretion understanding of how the Sec pathway Exportal, a finding that is consistent with a functioned and was organized in this model in which the targeting of the secretion A fundamental challenge confronted by all microorganism. Using both immunogold proteins results from the ability of the organisms concerns the successful secretion electron microscopy and fluorescence microdomain to accumulate high of biologically active proteins across cellular microscopy to probe for a secreted concentrations of the Sec translocons. membranes. Now, a report just published in streptococcal virulence factor (SpeB), the These data represent a new mechanism for Science details the discovery of one authors were able to demonstrate that asymmetric protein secretion using the Sec mechanism used by Gram-positive bacteria secretion occurred at a single microdomain in pathway, and constitute a significant advance to export proteins crucial to their survival the cellular membrane. The authors were in our knowledge of streptococcal protein and proliferation. further able to show the same targeted secretion. A more intriguing possibility is Unlike the periplasmic space of Gram- localization with a non-streptococcal protein that this secretion process might also negative bacteria or the endoplasmic (PhoZ), indicating that this secretion represent a paradigm for secretion common reticulum of eukaryotic cells, Gram-positive mechanism is a general phenomenon and is to all Gram-positive bacteria. Future work bacteria lack a specialized compartment not restricted to SpeB. Together, these will be needed to address this possibility as external to the cell membrane that allows findings clearly establish that protein well as the contributions of the Exportal to folding of secreted proteins. To address the secretion in S. pyogenes occurs at a distinct host–pathogen interactions. question of how these bacteria export microdomain of the cytoplasmic membrane David O’Connell proteins, Jason Rosch and Michael Caparon dedicated to protein export, a domain that the References and links focused on the secretion mechanisms of the authors have named the Exportal.As the ORIGINAL RESEARCH PAPER Rosch, J. & Caparon, M. important human pathogen, Streptococcus asymmetric secretion of molecules is essential A microdomain for protein secretion in Gram-positive pyogenes,a Gram-positive microorganism to many processes in bacteria, Rosch and bacteria. Science 304, 1513–1515 (2004) WEB SITE that secretes more than 40 proteins as part of Caparon further investigated the basis for this Michael Caparon’s laboratory: its pathogenic strategy. Genome analysis had targeted localization of secreted proteins. http://www.microbiology.wustl.edu/dept/fac/caparon.html 524 | JULY 2004 | VOLUME 2 www.nature.com/reviews/micro similar arrangement of structural fea- tures including β-sheets. Plus, in com- mon with the animal and bacterial viruses, there might be a lipid envelope present, which must be confirmed biochemically. The lack of ORFs that are con- served between viruses isolated from archaeal species has been puzzling because it might be expected that important genes encoding proteins that govern genome replication, for example, would be conserved. Perhaps parallel evolution in these ecologically isolated species gener- ated their incredible diversity. Going back to basics and using shape as a BACTERIAL PATHOGENICITY defining characteristic, like tradi- tional naturalists, has borne fruit for these researchers. By coupling genomics with structural biology, the Step by step lineages of the tree of life — even for viruses — might yet be defined. More than 50 years after the first bacterial type IV single-stranded product, forming a VirD2–T-DNA Susan Jones secretion system (T4SS) — the conjugation transfer intermediate. Cascales and Christie apparatus of the F-plasmid — was described, a analysed the sequential involvement of the VirB References and links ORIGINAL RESEARCH PAPER Rice, G. et al. new report in Science has defined the specific proteins in the translocation of the VirD2–T-DNA The structure of a thermophilic archaeal virus translocation pathway for a DNA substrate substrate using the quantitative TrIP assay and a shows a double-stranded DNA viral capsid type series of mutants each lacking a single VirB that spans all domains of life. Proc. Natl Acad. Sci. through a T4SS. USA 101, 7716–7720 (2004) Bacteria use T4SSs for a variety of fundamental protein. WEB SITE biological functions, including the exchange of The five VirB proteins that precipitated the Mark Young’s laboratory: http://www.homepage.montana.edu/ genetic material with other bacteria and the most T-DNA (VirB2, -B6, -B8, -B9 and -B11) are ~markyoung/default.htm translocation of oncogenic DNA and pathogenic classified by the authors as class I T4SS subunits, effector proteins into eukaryotic host cells. and it is proposed that they are distributed across Research on T4SSs has generated a large body of the cell envelope and form the channel of the knowledge regarding the genes and proteins secretory apparatus. The remaining six VirB involved, the structure of the secretory apparatus proteins were divided into two classes. For the class and the effects of the translocated substrates on III subunits (VirB1 and -B3 and the accessory host cells. Now, Cascales and Christie have gone protein VirJ), there was no evidence of contact with one step further and developed a method to the T-DNA. The class II subunits (VirB4, -B5, -B7 determine the sequential pathway of DNA and -B10) interacted with low amounts of T-DNA, translocation through the archetypal and evidence was presented that these subunits Agrobacterium tumefaciens VirB/D4 T4SS. probably precipitated T-DNA indirectly through Cascales and Christie developed a sensitive formaldehyde cross-linking to class I subunits. assay based on the chromatin The results of this sensitive, systematic analysis immunoprecipitation (ChIP) assay, a technique provide proof for the hypotheses of the that had previously been used to study specific contributions of individual subunits that had been protein–DNA binding in both eukaryotes and generated from previous studies. Cascales and prokaryotes. Cascales and Christie call their Christie conclude that ‘TrIP should also prove variation transfer DNA immunoprecipitation highly useful for studies of many other (TrIP), and the assay has three stages — fundamental processes…that involve the formaldehyde treatment of A. tumefaciens cells movement of DNA across biological membranes, induces in vivo cross-linking between DNA and including bacterial transformation and proteins, the proteins of interest are transduction…and viral infection cycles.’ immunoprecipitated and the co-precipitation of Sheilagh Clarkson DNA with the