title: Atomic structure of bacteriophage Sf6 tail needle knob.
abstract: Podoviridae are double-stranded DNA bacteriophages that use short, non-contractile tails to adsorb to the host cell surface. Within the tail apparatus of P22-like phages, a dedicated fiber known as the "tail needle" likely functions as a cell envelope-penetrating device to promote ejection of viral DNA inside the host. In Sf6, a P22-like phage that infects Shigella flexneri, the tail needle presents a C-terminal globular knob. This knob, absent in phage P22 but shared in other members of the P22-like genus, represents the outermost exposed tip of the virion that contacts the host cell surface. Here, we report a crystal structure of the Sf6 tail needle knob determined at 1.0 Å resolution. The structure reveals a trimeric globular domain of the TNF fold structurally superimposable with that of the tail-less phage PRD1 spike protein P5 and the adenovirus knob, domains that in both viruses function in receptor binding. However, P22-like phages are not known to utilize a protein receptor and are thought to directly penetrate the host surface. At 1.0 Å resolution, we identified three equivalents of l-glutamic acid (l-Glu) bound to each subunit interface. Although intimately bound to the protein, l-Glu does not increase the structural stability of the trimer nor it affects its ability to self-trimerize in vitro. In analogy to P22 gp26, we suggest the tail needle of phage Sf6 is ejected through the bacterial cell envelope during infection and its C-terminal knob is threaded through peptidoglycan pores formed by glycan strands.
authors.affiliation: Department of Biochemistry and Molecular Biology, Thomas Jefferson University, Philadelphia, Pennsylvania 19107.
authors.forename: Anshul
authors.initials: A
authors.lastname: Bhardwaj
authors.affiliation: Section of Molecular Genetics and Microbiology, University of Texas at Austin, Austin, Texas 78712.
authors.forename: Ian J
authors.initials: IJ
authors.lastname: Molineux
authors.affiliation: Department of Pathology, University of Utah School of Medicine, Salt Lake City, Utah 84112.
authors.forename: Sherwood R
authors.initials: SR
authors.lastname: Casjens
authors.affiliation: Department of Biochemistry and Molecular Biology, Thomas Jefferson University, Philadelphia, Pennsylvania 19107. Electronic address: gino.cingolani@jefferson.edu.
authors.forename: Gino
authors.initials: G
authors.lastname: Cingolani
mesh_terms: D000256:Adenoviridae / Q000378:metabolism; D001435:Bacteriophages / Q000737:chemistry* / Q000378:metabolism / Q000648:ultrastructure; D002453:Cell Cycle; D002843:Chromatin / Q000737:chemistry; D018360:Crystallography, X-Ray / Q000379:methods; D004279:DNA, Viral / Q000378:metabolism; D046529:Microscopy, Electron, Transmission / Q000379:methods; D008954:Models, Biological; D008958:Models, Molecular; D008968:Molecular Conformation; D020411:Oligonucleotide Array Sequence Analysis; D017434:Protein Structure, Tertiary; D012763:Shigella flexneri / Q000378:metabolism* / Q000821:virology; D014158:Transcription, Genetic; D017376:Viral Tail Proteins / Q000737:chemistry*
chemical_list: D002843:Chromatin; D004279:DNA, Viral; D017376:Viral Tail Proteins