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Prokhorov NS, Davis C, Maruthi K, Yang Q, Sherman M, Woodson M, White M, Miller LM, Jarrold M, Catalano C, Morais M. Biophysical and structural characterization of a multifunctional viral genome packaging motor. Nucleic Acids Res 2024; 52:831-843. [PMID: 38084901 PMCID: PMC10810279 DOI: 10.1093/nar/gkad1135] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/03/2023] [Revised: 11/08/2023] [Accepted: 11/10/2023] [Indexed: 01/26/2024] Open
Abstract
The large dsDNA viruses replicate their DNA as concatemers consisting of multiple covalently linked genomes. Genome packaging is catalyzed by a terminase enzyme that excises individual genomes from concatemers and packages them into preassembled procapsids. These disparate tasks are catalyzed by terminase alternating between two distinct states-a stable nuclease that excises individual genomes and a dynamic motor that translocates DNA into the procapsid. It was proposed that bacteriophage λ terminase assembles as an anti-parallel dimer-of-dimers nuclease complex at the packaging initiation site. In contrast, all characterized packaging motors are composed of five terminase subunits bound to the procapsid in a parallel orientation. Here, we describe biophysical and structural characterization of the λ holoenzyme complex assembled in solution. Analytical ultracentrifugation, small angle X-ray scattering, and native mass spectrometry indicate that 5 subunits assemble a cone-shaped terminase complex. Classification of cryoEM images reveals starfish-like rings with skewed pentameric symmetry and one special subunit. We propose a model wherein nuclease domains of two subunits alternate between a dimeric head-to-head arrangement for genome maturation and a fully parallel arrangement during genome packaging. Given that genome packaging is strongly conserved in both prokaryotic and eukaryotic viruses, the results have broad biological implications.
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Affiliation(s)
- Nikolai S Prokhorov
- Department of Biochemistry and Molecular Biology, The University of Texas Medical Branch at Galveston, Galveston, TX 77555, USA
- Sealy Center for Structural Biology, The University of Texas Medical Branch at Galveston, Galveston, TX 77555, USA
- Department of Molecular and Cellular Biochemistry, Indiana University, Bloomington, IN 47405, USA
| | - Christal R Davis
- Program in Structural Biology and Biochemistry, University of Colorado Anschutz Medical Campus, Aurora, CO 80045, USA
| | - Kashyap Maruthi
- Department of Biochemistry and Molecular Biology, The University of Texas Medical Branch at Galveston, Galveston, TX 77555, USA
- Sealy Center for Structural Biology, The University of Texas Medical Branch at Galveston, Galveston, TX 77555, USA
| | - Qin Yang
- Department of Pharmaceutical Chemistry, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of Colorado Anschutz Campus, Aurora, CO 80045, USA
| | - Michael B Sherman
- Department of Biochemistry and Molecular Biology, The University of Texas Medical Branch at Galveston, Galveston, TX 77555, USA
- Sealy Center for Structural Biology, The University of Texas Medical Branch at Galveston, Galveston, TX 77555, USA
| | - Michael Woodson
- Sealy Center for Structural Biology, The University of Texas Medical Branch at Galveston, Galveston, TX 77555, USA
| | - Mark A White
- Department of Biochemistry and Molecular Biology, The University of Texas Medical Branch at Galveston, Galveston, TX 77555, USA
- Sealy Center for Structural Biology, The University of Texas Medical Branch at Galveston, Galveston, TX 77555, USA
| | - Lohra M Miller
- Department of Chemistry, Indiana University, Bloomington, IN 47405, USA
| | - Martin F Jarrold
- Department of Chemistry, Indiana University, Bloomington, IN 47405, USA
| | - Carlos E Catalano
- Program in Structural Biology and Biochemistry, University of Colorado Anschutz Medical Campus, Aurora, CO 80045, USA
- Department of Pharmaceutical Chemistry, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of Colorado Anschutz Campus, Aurora, CO 80045, USA
| | - Marc C Morais
- Department of Biochemistry and Molecular Biology, The University of Texas Medical Branch at Galveston, Galveston, TX 77555, USA
- Sealy Center for Structural Biology, The University of Texas Medical Branch at Galveston, Galveston, TX 77555, USA
- Department of Molecular and Cellular Biochemistry, Indiana University, Bloomington, IN 47405, USA
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2
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Lokareddy RK, Hou CFD, Li F, Yang R, Cingolani G. Viral Small Terminase: A Divergent Structural Framework for a Conserved Biological Function. Viruses 2022; 14:v14102215. [PMID: 36298770 PMCID: PMC9611059 DOI: 10.3390/v14102215] [Citation(s) in RCA: 2] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/14/2022] [Revised: 10/03/2022] [Accepted: 10/05/2022] [Indexed: 11/07/2022] Open
Abstract
The genome packaging motor of bacteriophages and herpesviruses is built by two terminase subunits, known as large (TerL) and small (TerS), both essential for viral genome packaging. TerL structure, composition, and assembly to an empty capsid, as well as the mechanisms of ATP-dependent DNA packaging, have been studied in depth, shedding light on the chemo-mechanical coupling between ATP hydrolysis and DNA translocation. Instead, significantly less is known about the small terminase subunit, TerS, which is dispensable or even inhibitory in vitro, but essential in vivo. By taking advantage of the recent revolution in cryo-electron microscopy (cryo-EM) and building upon a wealth of crystallographic structures of phage TerSs, in this review, we take an inventory of known TerSs studied to date. Our analysis suggests that TerS evolved and diversified into a flexible molecular framework that can conserve biological function with minimal sequence and quaternary structure conservation to fit different packaging strategies and environmental conditions.
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Abstract
Although the process of genome encapsidation is highly conserved in tailed bacteriophages and eukaryotic double-stranded DNA viruses, there are two distinct packaging pathways that these viruses use to catalyze ATP-driven translocation of the viral genome into a preassembled procapsid shell. One pathway is used by ϕ29-like phages and adenoviruses, which replicate and subsequently package a monomeric, unit-length genome covalently attached to a virus/phage-encoded protein at each 5'-end of the dsDNA genome. In a second, more ubiquitous packaging pathway characterized by phage lambda and the herpesviruses, the viral DNA is replicated as multigenome concatemers linked in a head-to-tail fashion. Genome packaging in these viruses thus requires excision of individual genomes from the concatemer that are then translocated into a preassembled procapsid. Hence, the ATPases that power packaging in these viruses also possess nuclease activities that cut the genome from the concatemer at the beginning and end of packaging. This review focuses on proposed mechanisms of genome packaging in the dsDNA viruses using unit-length ϕ29 and concatemeric λ genome packaging motors as representative model systems.
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Affiliation(s)
- Carlos E Catalano
- Department of Pharmaceutical Sciences, Skaggs School of Pharmacy and Pharmaceutical Sciences, University of Colorado, Aurora, CO, United States.
| | - Marc C Morais
- Department of Biochemistry and Molecular Biology, Sealy Center for Structural and Molecular Biophysics, University of Texas Medical Branch at Galveston, Galveston, TX, United States
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4
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Yang Q, Catalano CE. ATP serves as a nucleotide switch coupling the genome maturation and packaging motor complexes of a virus assembly machine. Nucleic Acids Res 2020; 48:5006-5015. [PMID: 32255177 PMCID: PMC7229814 DOI: 10.1093/nar/gkaa205] [Citation(s) in RCA: 5] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/21/2019] [Revised: 03/10/2020] [Accepted: 04/02/2020] [Indexed: 11/29/2022] Open
Abstract
The assembly of double-stranded DNA viruses, from phages to herpesviruses, is strongly conserved. Terminase enzymes processively excise and package monomeric genomes from a concatemeric DNA substrate. The enzymes cycle between a stable maturation complex that introduces site-specific nicks into the duplex and a dynamic motor complex that rapidly translocates DNA into a procapsid shell, fueled by ATP hydrolysis. These tightly coupled reactions are catalyzed by terminase assembled into two functionally distinct nucleoprotein complexes; the maturation complex and the packaging motor complex, respectively. We describe the effects of nucleotides on the assembly of a catalytically competent maturation complex on viral DNA, their effect on maturation complex stability and their requirement for the transition to active packaging motor complex. ATP plays a major role in regulating all of these activities and may serve as a 'nucleotide switch' that mediates transitions between the two complexes during processive genome packaging. These biological processes are recapitulated in all of the dsDNA viruses that package monomeric genomes from concatemeric DNA substrates and the nucleotide switch mechanism may have broad biological implications with respect to virus assembly mechanisms.
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Affiliation(s)
- Qin Yang
- Skaggs School of Pharmacy and Pharmaceutical Sciences, Department of Pharmaceutical Chemistry, University of Colorado Anschutz Medical Campus, Aurora, CO 80045, USA
| | - Carlos E Catalano
- Skaggs School of Pharmacy and Pharmaceutical Sciences, Department of Pharmaceutical Chemistry, University of Colorado Anschutz Medical Campus, Aurora, CO 80045, USA
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5
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delToro D, Ortiz D, Ordyan M, Pajak J, Sippy J, Catala A, Oh CS, Vu A, Arya G, Smith DE, Catalano CE, Feiss M. Functional Dissection of a Viral DNA Packaging Machine's Walker B Motif. J Mol Biol 2019; 431:4455-4474. [PMID: 31473160 PMCID: PMC7416571 DOI: 10.1016/j.jmb.2019.08.012] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/28/2019] [Revised: 08/09/2019] [Accepted: 08/19/2019] [Indexed: 11/30/2022]
Abstract
Many viruses employ ATP-powered motors for genome packaging. We combined genetic, biochemical, and single-molecule techniques to confirm the predicted Walker-B ATP-binding motif in the phage λ motor and to investigate the roles of the conserved residues. Most changes of the conserved hydrophobic residues resulted in >107-fold decrease in phage yield, but we identified nine mutants with partial activity. Several were cold-sensitive, suggesting that mobility of the residues is important. Single-molecule measurements showed that the partially active A175L exhibits a small reduction in motor velocity and increase in slipping, consistent with a slowed ATP binding transition, whereas G176S exhibits decreased slipping, consistent with an accelerated transition. All changes to the conserved D178, predicted to coordinate Mg2+•ATP, were lethal except conservative change D178E. Biochemical interrogation of the inactive D178N protein found no folding or assembly defects and near-normal endonuclease activity, but a ∼200-fold reduction in steady-state ATPase activity, a lag in the single-turnover ATPase time course, and no DNA packaging, consistent with a critical role in ATP-coupled DNA translocation. Molecular dynamics simulations of related enzymes suggest that the aspartate plays an important role in enhancing the catalytic activity of the motor by bridging the Walker motifs and precisely contributing its charged group to help polarize the bound nucleotide. Supporting this prediction, single-molecule measurements revealed that change D178E reduces motor velocity without increasing slipping, consistent with a slowed hydrolysis step. Our studies thus illuminate the mechanistic roles of Walker-B residues in ATP binding, hydrolysis, and DNA translocation by this powerful motor.
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Affiliation(s)
- Damian delToro
- Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA
| | - David Ortiz
- Department of Medicinal Chemistry, University of Washington, Seattle, WA 98195, USA
| | - Mariam Ordyan
- Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA
| | - Joshua Pajak
- Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA
| | - Jean Sippy
- Department of Microbiology, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA
| | - Alexis Catala
- Department of Medicinal Chemistry, University of Washington, Seattle, WA 98195, USA
| | - Choon-Seok Oh
- Department of Microbiology, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA
| | - Amber Vu
- Department of Microbiology, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA
| | - Gaurav Arya
- Department of Mechanical Engineering and Materials Science, Duke University, Durham, NC 27708, USA
| | - Douglas E Smith
- Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA.
| | - Carlos E Catalano
- Department of Medicinal Chemistry, University of Washington, Seattle, WA 98195, USA.
| | - Michael Feiss
- Department of Microbiology, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA.
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6
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Yang TC, Ortiz D, Yang Q, De Angelis RW, Sanyal SJ, Catalano CE. Physical and Functional Characterization of a Viral Genome Maturation Complex. Biophys J 2017; 112:1551-1560. [PMID: 28445747 DOI: 10.1016/j.bpj.2017.02.041] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/06/2016] [Revised: 01/27/2017] [Accepted: 02/21/2017] [Indexed: 10/19/2022] Open
Abstract
Genome packaging is strongly conserved in the complex double-stranded DNA viruses, including the herpesviruses and many bacteriophages. In these cases, viral DNA is packaged into a procapsid shell by a terminase enzyme. The packaging substrate is typically a concatemer composed of multiple genomes linked in a head-to-tail fashion, and terminase enzymes perform two essential functions: 1) excision of a unit length genome from the concatemer (genome maturation) and 2) translocation of the duplex into a procapsid (genome packaging). While the packaging motors have been described in some detail, the maturation complexes remain ill characterized. Here we describe the assembly, physical characteristics, and catalytic activity of the λ-genome maturation complex. The λ-terminase protomer is composed of one large catalytic subunit tightly associated with two DNA recognition subunits. The isolated protomer binds DNA weakly and does not discriminate between nonspecific DNA and duplexes that contain the packaging initiation sequence, cos. The Escherichia coli integration host factor protein (IHF) is required for efficient λ-development in vivo and a specific IHF recognition sequence is found within cos. We show that IHF and the terminase protomer cooperatively assemble at the cos site and that the small terminase subunit plays the dominant role in complex assembly. Analytical ultracentrifugation analysis reveals that the maturation complex is composed of four protomers and one IHF heterodimer bound at the cos site. Tetramer assembly activates the cos-cleavage nuclease activity of the enzyme, which matures the genome end in preparation for packaging. The stoichiometry and catalytic activity of the complex is reminiscent of the type IIE and IIF restriction endonucleases and the two systems may share mechanistic features. This study, to our knowledge, provides our first detailed glimpse into the structural and functional features of a viral genome maturation complex, an essential intermediate in the development of complex dsDNA viruses.
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Affiliation(s)
- Teng-Chieh Yang
- Department of Medicinal Chemistry, School of Pharmacy, University of Washington, Seattle, Washington
| | - David Ortiz
- Department of Medicinal Chemistry, School of Pharmacy, University of Washington, Seattle, Washington
| | - Qin Yang
- Skaggs School of Pharmacy and Pharmaceutical Sciences, University of Colorado, Aurora, Colorado
| | - Rolando W De Angelis
- Skaggs School of Pharmacy and Pharmaceutical Sciences, University of Colorado, Aurora, Colorado
| | - Saurarshi J Sanyal
- Department of Medicinal Chemistry, School of Pharmacy, University of Washington, Seattle, Washington
| | - Carlos E Catalano
- Skaggs School of Pharmacy and Pharmaceutical Sciences, University of Colorado, Aurora, Colorado.
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7
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Hilbert BJ, Hayes JA, Stone NP, Xu RG, Kelch BA. The large terminase DNA packaging motor grips DNA with its ATPase domain for cleavage by the flexible nuclease domain. Nucleic Acids Res 2017; 45:3591-3605. [PMID: 28082398 PMCID: PMC5389665 DOI: 10.1093/nar/gkw1356] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/13/2016] [Accepted: 01/05/2017] [Indexed: 01/07/2023] Open
Abstract
Many viruses use a powerful terminase motor to pump their genome inside an empty procapsid shell during virus maturation. The large terminase (TerL) protein contains both enzymatic activities necessary for packaging in such viruses: the adenosine triphosphatase (ATPase) that powers DNA translocation and an endonuclease that cleaves the concatemeric genome at both initiation and completion of genome packaging. However, how TerL binds DNA during translocation and cleavage remains mysterious. Here we investigate DNA binding and cleavage using TerL from the thermophilic phage P74-26. We report the structure of the P74-26 TerL nuclease domain, which allows us to model DNA binding in the nuclease active site. We screened a large panel of TerL variants for defects in binding and DNA cleavage, revealing that the ATPase domain is the primary site for DNA binding, and is required for nuclease activity. The nuclease domain is dispensable for DNA binding but residues lining the active site guide DNA for cleavage. Kinetic analysis of DNA cleavage suggests flexible tethering of the nuclease domains during DNA cleavage. We propose that interactions with the procapsid during DNA translocation conformationally restrict the nuclease domain, inhibiting cleavage; TerL release from the capsid upon completion of packaging unlocks the nuclease domains to cleave DNA.
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Affiliation(s)
- Brendan J. Hilbert
- Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA
| | - Janelle A. Hayes
- Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA
| | - Nicholas P. Stone
- Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA
| | - Rui-Gang Xu
- York Structural Biology Laboratory, Department of Chemistry, University of York, York YO10 5DD, UK
| | - Brian A. Kelch
- Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605, USA,To whom correspondence should be addressed. Tel: +1 508 856 8322; Fax: +1 508 856 6464;
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8
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Lambert S, Yang Q, De Angeles R, Chang JR, Ortega M, Davis C, Catalano CE. Molecular Dissection of the Forces Responsible for Viral Capsid Assembly and Stabilization by Decoration Proteins. Biochemistry 2017; 56:767-778. [DOI: 10.1021/acs.biochem.6b00705] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Affiliation(s)
- Shannon Lambert
- Department
of Medicinal Chemistry, School of Pharmacy, University of Washington, Box 357610, Seattle, Washington 98195, United States
| | - Qin Yang
- Skaggs
School of Pharmacy and Pharmaceutical Sciences, University of Colorado, Anschutz Medical Campus, Mail Stop C238, Aurora, Colorado 80045, United States
| | - Rolando De Angeles
- Skaggs
School of Pharmacy and Pharmaceutical Sciences, University of Colorado, Anschutz Medical Campus, Mail Stop C238, Aurora, Colorado 80045, United States
| | - Jenny R. Chang
- Skaggs
School of Pharmacy and Pharmaceutical Sciences, University of Colorado, Anschutz Medical Campus, Mail Stop C238, Aurora, Colorado 80045, United States
| | - Marcos Ortega
- Department
of Biology, Macalester College, St. Paul, Minnesota 55105, United States
| | - Christal Davis
- Program
in Structural Biology and Biochemistry, University of Colorado, Anschutz Medical Campus, Mail Stop C290, Aurora, Colorado 80045, United States
| | - Carlos Enrique Catalano
- Department
of Medicinal Chemistry, School of Pharmacy, University of Washington, Box 357610, Seattle, Washington 98195, United States
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9
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Yang TC, Langford AJ, Kumar S, Ruesch JC, Wang W. Trimerization Dictates Solution Opalescence of a Monoclonal Antibody. J Pharm Sci 2016; 105:2328-37. [DOI: 10.1016/j.xphs.2016.05.027] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/03/2015] [Revised: 05/08/2016] [Accepted: 05/25/2016] [Indexed: 11/25/2022]
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10
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delToro D, Ortiz D, Ordyan M, Sippy J, Oh CS, Keller N, Feiss M, Catalano CE, Smith DE. Walker-A Motif Acts to Coordinate ATP Hydrolysis with Motor Output in Viral DNA Packaging. J Mol Biol 2016; 428:2709-29. [PMID: 27139643 PMCID: PMC4905814 DOI: 10.1016/j.jmb.2016.04.029] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/03/2016] [Revised: 04/15/2016] [Accepted: 04/23/2016] [Indexed: 10/21/2022]
Abstract
During the assembly of many viruses, a powerful ATP-driven motor translocates DNA into a preformed procapsid. A Walker-A "P-loop" motif is proposed to coordinate ATP binding and hydrolysis with DNA translocation. We use genetic, biochemical, and biophysical techniques to survey the roles of P-loop residues in bacteriophage lambda motor function. We identify 55 point mutations that reduce virus yield to below detectable levels in a highly sensitive genetic complementation assay and 33 that cause varying reductions in yield. Most changes in the predicted conserved residues K76, R79, G81, and S83 produce no detectable yield. Biochemical analyses show that R79A and S83A mutant proteins fold, assemble, and display genome maturation activity similar to wild-type (WT) but exhibit little ATPase or DNA packaging activity. Kinetic DNA cleavage and ATPase measurements implicate R79 in motor ring assembly on DNA, supporting recent structural models that locate the P-loop at the interface between motor subunits. Single-molecule measurements detect no translocation for K76A and K76R, while G81A and S83A exhibit strong impairments, consistent with their predicted roles in ATP binding. We identify eight residue changes spanning A78-K84 that yield impaired translocation phenotypes and show that Walker-A residues play important roles in determining motor velocity, pausing, and processivity. The efficiency of initiation of packaging correlates strongly with motor velocity. Frequent pausing and slipping caused by changes A78V and R79K suggest that these residues are important for ATP alignment and coupling of ATP binding to DNA gripping. Our findings support recent structural models implicating the P-loop arginine in ATP hydrolysis and mechanochemical coupling.
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Affiliation(s)
- Damian delToro
- Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA
| | - David Ortiz
- Department of Medicinal Chemistry, University of Washington, Seattle, WA 98195, USA
| | - Mariam Ordyan
- Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA
| | - Jean Sippy
- Department of Microbiology, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA
| | - Choon-Seok Oh
- Department of Microbiology, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA
| | - Nicholas Keller
- Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA
| | - Michael Feiss
- Department of Microbiology, Roy J. and Lucille A. Carver College of Medicine, University of Iowa, Iowa City, IA 52242, USA.
| | - Carlos E Catalano
- Department of Medicinal Chemistry, University of Washington, Seattle, WA 98195, USA.
| | - Douglas E Smith
- Department of Physics, University of California, San Diego, La Jolla, CA 92093, USA.
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11
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Abstract
During progeny assembly, viruses selectively package virion genomes from a nucleic acid pool that includes host nucleic acids. For large dsDNA viruses, including tailed bacteriophages and herpesviruses, immature viral DNA is recognized and translocated into a preformed icosahedral shell, the prohead. Recognition involves specific interactions between the viral packaging enzyme, terminase, and viral DNA recognition sites. Generally, viral DNA is recognized by terminase’s small subunit (TerS). The large terminase subunit (TerL) contains translocation ATPase and endonuclease domains. In phage lambda, TerS binds a sequence repeated three times in cosB, the recognition site. TerS binding to cosB positions TerL to cut the concatemeric DNA at the adjacent nicking site, cosN. TerL introduces staggered nicks in cosN, generating twelve bp cohesive ends. Terminase separates the cohesive ends and remains bound to the cosB-containing end, in a nucleoprotein structure called Complex I. Complex I docks on the prohead’s portal vertex and translocation ensues. DNA topology plays a role in the TerSλ-cosBλ interaction. Here we show that a site, I2, located between cosN and cosB, is critically important for an early DNA packaging step. I2 contains a complex static bend. I2 mutations block DNA packaging. I2 mutant DNA is cut by terminase at cosN in vitro, but in vivo, no cos cleavage is detected, nor is there evidence for Complex I. Models for what packaging step might be blocked by I2 mutations are presented.
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