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Jana AK, May ER. Structural and dynamic asymmetry in icosahedrally symmetric virus capsids. Curr Opin Virol 2020; 45:8-16. [PMID: 32615360 PMCID: PMC7746594 DOI: 10.1016/j.coviro.2020.06.002] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/12/2020] [Revised: 05/30/2020] [Accepted: 06/04/2020] [Indexed: 02/06/2023]
Abstract
A common characteristic of virus capsids is icosahedral symmetry, yet these highly symmetric structures can display asymmetric features within their virions and undergo asymmetric dynamics. The fields of structural and computational biology have entered a new realm in the investigation of virus infection mechanisms, with the ability to observe symmetry-breaking features. This review will cover important studies on icosahedral virus structure and dynamics, covering both symmetric and asymmetric conformational changes. However, the main emphasis of the review will be towards recent studies employing cryo-electron microscopy or molecular dynamics simulations, which can uncover asymmetric aspects of these systems relevant to understanding viral physical-chemical properties and their biological impact.
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Affiliation(s)
- Asis K Jana
- Department of Molecular and Cell Biology, University of Connecticut, Storrs, CT 06269, USA
| | - Eric R May
- Department of Molecular and Cell Biology, University of Connecticut, Storrs, CT 06269, USA.
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2
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Khaykelson D, Raviv U. Studying viruses using solution X-ray scattering. Biophys Rev 2020; 12:41-48. [PMID: 32062837 PMCID: PMC7040123 DOI: 10.1007/s12551-020-00617-4] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/16/2019] [Accepted: 01/07/2020] [Indexed: 12/23/2022] Open
Abstract
Viruses have been of interest to mankind since their discovery as small infectious agents in the nineteenth century. Because many viruses cause diseases to humans and agriculture, they were rigorously studied for biological and medical purposes. Viruses have remarkable properties such as the symmetry and self-assembly of their protein envelope, maturation into infectious virions, structural stability, and disassembly. Solution X-ray scattering can probe structures and reactions in solutions, down to subnanometer spatial resolution and millisecond temporal resolution. It probes the bulk solution and reveals the average shape and average mass of particles in solution and can be used to study kinetics and thermodynamics of viruses at different stages of their life cycle. Here we review recent work that demonstrates the capabilities of solution X-ray scattering to study in vitro the viral life cycle.
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Affiliation(s)
- Daniel Khaykelson
- Institute of Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem, 9190401, Israel.
| | - Uri Raviv
- Institute of Chemistry, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem, 9190401, Israel.
- Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem, 9190401, Israel.
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3
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May ER, Arora K, Brooks CL. pH-induced stability switching of the bacteriophage HK97 maturation pathway. J Am Chem Soc 2014; 136:3097-107. [PMID: 24495192 PMCID: PMC3985869 DOI: 10.1021/ja410860n] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Abstract
Many viruses undergo large-scale conformational changes during their life cycles. Blocking the transition from one stage of the life cycle to the next is an attractive strategy for the development of antiviral compounds. In this work, we have constructed an icosahedrally symmetric, low-energy pathway for the maturation transition of bacteriophage HK97. By conducting constant-pH molecular dynamics simulations on this pathway, we identify which residues are contributing most significantly to shifting the stability between the states along the pathway under differing pH conditions. We further analyze these data to establish the connection between critical residues and important structural motifs which undergo reorganization during maturation. We go on to show how DNA packaging can induce spontaneous reorganization of the capsid during maturation.
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Affiliation(s)
- Eric R May
- Department of Molecular and Cell Biology, University of Connecticut , Storrs, Connecticut 06269, United States
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4
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Stuart DI, Abrescia NGA. From lows to highs: using low-resolution models to phase X-ray data. ACTA CRYSTALLOGRAPHICA. SECTION D, BIOLOGICAL CRYSTALLOGRAPHY 2013; 69:2257-65. [PMID: 24189238 PMCID: PMC3817700 DOI: 10.1107/s0907444913022336] [Citation(s) in RCA: 11] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/01/2013] [Accepted: 08/08/2013] [Indexed: 11/11/2022]
Abstract
The study of virus structures has contributed to methodological advances in structural biology that are generally applicable (molecular replacement and noncrystallographic symmetry are just two of the best known examples). Moreover, structural virology has been instrumental in forging the more general concept of exploiting phase information derived from multiple structural techniques. This hybridization of structural methods, primarily electron microscopy (EM) and X-ray crystallography, but also small-angle X-ray scattering (SAXS) and nuclear magnetic resonance (NMR) spectroscopy, is central to integrative structural biology. Here, the interplay of X-ray crystallography and EM is illustrated through the example of the structural determination of the marine lipid-containing bacteriophage PM2. Molecular replacement starting from an ~13 Å cryo-EM reconstruction, followed by cycling density averaging, phase extension and solvent flattening, gave the X-ray structure of the intact virus at 7 Å resolution This in turn served as a bridge to phase, to 2.5 Å resolution, data from twinned crystals of the major coat protein (P2), ultimately yielding a quasi-atomic model of the particle, which provided significant insights into virus evolution and viral membrane biogenesis.
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Affiliation(s)
- David I. Stuart
- Division of Structural Biology, The Wellcome Trust Centre for Human Genetics, University of Oxford, Roosevelt Drive, Headington, Oxford OX3 7BN, England
- Diamond Light Source Ltd, Diamond House, Harwell Science and Innovation Campus, Didcot, England
| | - Nicola G. A. Abrescia
- Structural Biology Unit, CIC bioGUNE, CIBERehd, Bizkaia Technology Park, Bld 800, 48160 Derio, Spain
- IKERBASQUE, Basque Foundation for Science, 48011 Bilbao, Spain
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5
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Functional domains of the HK97 capsid maturation protease and the mechanisms of protein encapsidation. J Mol Biol 2013; 425:2765-81. [PMID: 23688818 DOI: 10.1016/j.jmb.2013.05.002] [Citation(s) in RCA: 25] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/07/2013] [Revised: 05/08/2013] [Accepted: 05/09/2013] [Indexed: 01/19/2023]
Abstract
Tailed double-stranded DNA bacteriophages and herpesviruses build capsids by co-assembling a major capsid protein with an internal scaffolding protein that then exits from the assembled structure either intact or after digestion in situ by a protease. In bacteriophage HK97, the 102-residue N-terminal delta domain of the major capsid protein is also removed by proteolysis after assembly and appears to perform the scaffolding function. We describe the HK97 protease that carries out these maturation cleavages. Insertion mutations at seven sites in the protease gene produced mutant proteins that assemble into proheads, and those in the N-terminal two-thirds were enzymatically inactive. Plasmid-expressed protease was rapidly cleaved in vivo but was stabilized by co-expression with the delta domain. Purified protease was found to be active during the assembly of proheads in vitro. Heterologous fusions to the intact protease or to C-terminal fragments targeted fusion proteins into proheads. We confirm that the catalytic activity resides in the N-terminal two-thirds of the protease polypeptide and suggest that the C-terminal one-fifth of the protein contains a capsid targeting signal. The implications of this arrangement are compared to capsid targeting systems in other phages, herpesviruses, and encapsulins.
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6
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Neira JL. Fluorescence, circular dichroism and mass spectrometry as tools to study virus structure. Subcell Biochem 2013; 68:177-202. [PMID: 23737052 DOI: 10.1007/978-94-007-6552-8_6] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 06/02/2023]
Abstract
Fluorescence and circular dichroism, as analytical spectroscopic techniques, and mass spectrometry as an analytical tool to determine the molecular mass, provide important biophysical approaches in structural virology. Although they do not provide atomic, or near-atomic, details as electron microscopy, X-ray crystallography or nuclear magnetic resonance spectroscopy can do, they do provide important insights into virus particle composition, structure, conformational stability and dynamics, assembly and maturation, and interactions with other viral and cellular biomolecules. They can be used also to investigate the molecular determinants of virus particle structure and properties, and the changes induced in them by external factors. In this chapter, I describe the physical bases of these three techniques, and some examples on how they have helped us to understand virus particle structure and physicochemical properties.
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Affiliation(s)
- José L Neira
- Instituto de Biología Molecular y Celular, Universidad Miguel Hernández, 03202, Elche, Alicante, Spain,
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7
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Abstract
We examine virus maturation of selected nonenveloped and enveloped single-stranded RNA viruses, retroviruses, bacteriophages, and herpesviruses. Processes associated with maturation in the RNA viruses range from subtle (nodaviruses and picornaviruses) to dramatic (tetraviruses and togaviruses). The elaborate assembly and maturation pathway of HIV is discussed in contrast to the less sophisticated but highly efficient processes associated with togaviruses. Bacteriophage assembly and maturation are discussed in general terms, with specific examples chosen for emphasis. Finally the herpesviruses are compared with bacteriophages. The data support divergent evolution of nodaviruses, picornaviruses, and tetraviruses from a common ancestor and divergent evolution of alphaviruses and flaviviruses from a common ancestor. Likewise, bacteriophages and herpesviruses almost certainly share a common ancestor in their evolution. Comparing all the viruses, we conclude that maturation is a convergent process that is required to solve conflicting requirements in biological dynamics and function.
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Affiliation(s)
- David Veesler
- Department of Molecular Biology, The Scripps Research Institute, La Jolla, California 92037, USA.
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8
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Li X, Wu B, Liu Y, Pynn R, Shew CY, Smith GS, Herwig KW, Robertson JL, Chen WR, Liu L. Contrast variation in spin-echo small angle neutron scattering. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2012; 24:064115. [PMID: 22277831 DOI: 10.1088/0953-8984/24/6/064115] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/31/2023]
Abstract
The use of contrast variation in spin-echo small angle neutron scattering (SESANS) experiments is discussed for the case of colloidal structural investigation. On the basis of calculations for several model systems, we find that the contrast variation SESANS technique, in terms of the measured SESANS correlation function G(z), is not sensitive to the structural characteristics of colloidal suspensions consisting of particles with uniform scattering length density profiles. However, its ability to resolve structural heterogeneity, at both intra-colloidal and inter-colloidal length scales, is clearly demonstrated. The prospect of using this new technique to investigate structural information that is difficult to probe in other ways is also explored.
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Affiliation(s)
- Xin Li
- Soft Matter Thrust, Neutron Scattering Science Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA
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9
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Li X, Shew CY, He L, Meilleur F, Myles DAA, Liu E, Zhang Y, Smith GS, Herwig KW, Pynn R, Chen WR. Scattering functions of Platonic solids. J Appl Crystallogr 2011. [DOI: 10.1107/s0021889811011691] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/10/2022] Open
Abstract
The single-particle small-angle scattering properties of five Platonic solids, including the tetrahedron, hexahedron, octahedron, dodecahedron and icosahedron, are systematically investigated. For each given geometry, the Debye spatial autocorrelation function, pair distance distribution function and intraparticle structure factor (form factor) are calculated and compared with the corresponding scattering function of a spherical reference system. From the theoretical models, the empirical relationship between the dodecahedral and icosahedral structural characteristics and those of the equivalent spheres is found. Moreover, the single-particle scattering properties of icosahedral and spherical shells with identical volume are investigated, and the prospect of using different data analysis approaches to explore their structural differences is presented and discussed.
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10
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Huang RK, Khayat R, Lee KK, Gertsman I, Duda RL, Hendrix RW, Johnson JE. The Prohead-I structure of bacteriophage HK97: implications for scaffold-mediated control of particle assembly and maturation. J Mol Biol 2011; 408:541-54. [PMID: 21276801 DOI: 10.1016/j.jmb.2011.01.016] [Citation(s) in RCA: 39] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2010] [Revised: 01/02/2011] [Accepted: 01/07/2011] [Indexed: 10/18/2022]
Abstract
Virus capsid assembly requires recruiting and organizing multiple copies of protein subunits to form a closed shell for genome packaging that leads to infectivity. Many viruses encode scaffolding proteins to shift the equilibrium toward particle formation by promoting intersubunit interactions and stabilizing assembly intermediates. Bacteriophage HK97 lacks an explicit scaffolding protein, but the capsid protein (gp5) contains a scaffold-like N-terminal segment termed the delta domain. When gp5 is expressed in Escherichia coli, the delta domain guides 420 copies of the subunit into a procapsid with T=7 laevo icosahedral symmetry named Prohead-I. Prohead-I can be disassembled and reassembled under mild conditions and it cannot mature further. When the virally encoded protease (gp4) is coexpressed with gp5, it is incorporated into the capsid and digests the delta domain followed by autoproteolysis to produce the metastable Prohead-II. Prohead-I(+P) was isolated by coexpressing gp5 and an inactive mutant of gp4. Prohead-I and Prohead-I(+P) were compared by biochemical methods, revealing that the inactive protease stabilized the capsid against disassembly by chemical or physical stress. The crystal structure of Prohead-I(+P) was determined at 5.2 Å resolution, and distortions were observed in the subunit tertiary structures similar to those observed previously in Prohead-II. Prohead-I(+P) differed from Prohead-II due to the presence of the delta domain and the resulting repositioning of the N-arms, explaining why Prohead-I can be reversibly dissociated and cannot mature. Low-resolution X-ray data enhanced the density of the relatively dynamic delta domains, revealing their quaternary arrangement and suggesting how they drive proper assembly.
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Affiliation(s)
- Rick K Huang
- Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA
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11
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Miao Y, Johnson JE, Ortoleva PJ. All-atom multiscale simulation of cowpea chlorotic mottle virus capsid swelling. J Phys Chem B 2010; 114:11181-95. [PMID: 20695471 DOI: 10.1021/jp102314e] [Citation(s) in RCA: 50] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/27/2022]
Abstract
An all-atom multiscale computational modeling approach, molecular dynamics/order parameter extrapolation (MD/OPX), has recently been developed for simulating large bionanosystems. It accelerates MD simulations and addresses rapid atomistic fluctuations and slowly varying nanoscale dynamics of bionanosystems simultaneously. With modules added to account for water molecules and ions, MD/OPX is applied to simulate the swelling of cowpea chlorotic mottle virus (CCMV) capsid solvated in a host medium in this study. Simulation results show that the N-terminal arms of capsid proteins undergo large deviations from the initial configurations with their length extended quickly during the early stage of capsid swelling. The capsid swelling is a symmetry-breaking process involving local initiation and front propagation. The capsid swelling rate is approximately 0.25 nm/ns (npn) during the early stage of the simulation, and propagation of the structural transition across the capsid is roughly 0.6 npn. The system conditions that affect swelling of the capsid are analyzed. Prospects for creating a phase diagram for CCMV capsid swelling and using predictions to guide experiments are discussed.
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Affiliation(s)
- Yinglong Miao
- Center for Cell and Virus Theory, Department of Chemistry, Indiana University, Bloomington, Indiana 47405, USA
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12
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Johnson JE. Virus particle maturation: insights into elegantly programmed nanomachines. Curr Opin Struct Biol 2010; 20:210-6. [PMID: 20149636 PMCID: PMC2854226 DOI: 10.1016/j.sbi.2010.01.004] [Citation(s) in RCA: 78] [Impact Index Per Article: 5.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/20/2009] [Revised: 01/07/2010] [Accepted: 01/08/2010] [Indexed: 10/19/2022]
Abstract
Similar modes of virus maturation have been observed in dsDNA bacteriophages and the structurally related herpes viruses and some type of maturation occur in most animal viruses. Recently a variety of biophysical studies of maturation intermediates of bacteriophages P22, lambda, and HK97 have suggested an energy landscape that drives the transitions and structure-based mechanisms for its formation. Near-atomic resolution models of subunit tertiary structures in an early intermediate of bacteriophage HK97 maturation revealed a remarkable distortion of the secondary structures when compared to the mature particle. Scaffolding proteins may induce the distortion that is maintained by quaternary structure interactions following scaffold release, making the intermediate particle metastable.
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Affiliation(s)
- John E Johnson
- Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA.
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13
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Gertsman I, Fu CY, Huang R, Komives EA, Johnson JE. Critical salt bridges guide capsid assembly, stability, and maturation behavior in bacteriophage HK97. Mol Cell Proteomics 2010; 9:1752-63. [PMID: 20332083 DOI: 10.1074/mcp.m000039-mcp201] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
HK97 is a double-stranded DNA bacteriophage that undergoes dramatic conformational changes during viral capsid maturation and for which x-ray structures, at near atomic resolution, of multiple intermediate and mature capsid states are available. Both amide H/(2)H exchange and crystallographic comparisons between the pre-expanded Prohead II particles and the expanded Head II of bacteriophage HK97 revealed quaternary interactions that remain fixed throughout maturation and appear to maintain intercapsomer integrity at all quasi- and icosahedral 3-fold axes. These 3-fold staples are formed from Arg and Glu residues and a metal binding site. Mutations of either Arg-347 or Arg-194 or a double mutation of E344Q and E363A resulted in purification of the phage in capsomer form (hexamers and pentamers). Mutants that did assemble had both decreased thermal stability and decreased in vitro expansion rates. Amide H/(2)H exchange mass spectrometry showed that in the wild type capsid some subunits had a bent "spine" helix (highly exchanging), whereas others were straight (less exchanging). Similar analysis of the never assembled mutant capsomers showed uniform amide exchange in all of these that was higher than that of the straight spine helices (characterized in more mature intermediates), suggesting that the spine helix is somewhat bent prior to capsid assembly. The result further supports a previously proposed mechanism for capsid expansion in which the delta domains of each subunit induce a high energy intermediate conformation, which now appears to include a bent helix during capsomer assembly.
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Affiliation(s)
- Ilya Gertsman
- Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA
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14
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Gertsman I, Komives EA, Johnson JE. HK97 maturation studied by crystallography and H/2H exchange reveals the structural basis for exothermic particle transitions. J Mol Biol 2010; 397:560-74. [PMID: 20093122 DOI: 10.1016/j.jmb.2010.01.016] [Citation(s) in RCA: 23] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/18/2009] [Revised: 12/29/2009] [Accepted: 01/07/2010] [Indexed: 12/27/2022]
Abstract
HK97 is an exceptionally amenable system for characterizing major conformational changes associated with capsid maturation in double-stranded DNA bacteriophage. HK97 undergoes a capsid expansion of approximately 20%, accompanied by major subunit rearrangements during genome packaging. A previous 3.44-A-resolution crystal structure of the mature capsid Head II and cryo-electron microscopy studies of other intermediate expansion forms of HK97 suggested that, primarily, rigid-body movements facilitated the maturation process. We recently reported a 3.65-A-resolution structure of the preexpanded particle form Prohead II (P-II) and found that the capsid subunits undergo significant refolding and twisting of the tertiary structure to accommodate expansion. The P-II study focused on major twisting motions in the P-domain and on refolding of the spine helix during the transition. Here we extend the crystallographic comparison between P-II and Head II, characterizing the refolding events occurring in each of the four major domains of the capsid subunit and their effect on quaternary structure stabilization. In addition, hydrogen/deuterium exchange, coupled to mass spectrometry, was used to characterize the structural dynamics of three distinct capsid intermediates: P-II, Expansion Intermediate, and the nearly mature Head I. Differences in the solvent accessibilities of the seven quasi-equivalent capsid subunits, attributed to differences in secondary and quaternary structures, were observed in P-II. Nearly all differences in solvent accessibility among subunits disappear after the first transition to Expansion Intermediate. We show that most of the refolding is coupled to this transformation, an event associated with the transition from asymmetric to symmetric hexamers.
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Affiliation(s)
- Ilya Gertsman
- Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA
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15
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Miao Y, Ortoleva PJ. Viral structural transition mechanisms revealed by multiscale molecular dynamics/order parameter extrapolation simulation. Biopolymers 2010; 93:61-73. [DOI: 10.1002/bip.21299] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
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16
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Gertsman I, Gan L, Guttman M, Lee K, Speir JA, Duda RL, Hendrix RW, Komives EA, Johnson JE. An unexpected twist in viral capsid maturation. Nature 2009; 458:646-50. [PMID: 19204733 DOI: 10.1038/nature07686] [Citation(s) in RCA: 107] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/25/2008] [Accepted: 12/08/2008] [Indexed: 11/09/2022]
Abstract
Lambda-like double-stranded (ds) DNA bacteriophage undergo massive conformational changes in their capsid shell during the packaging of their viral genomes. Capsid shells are complex organizations of hundreds of protein subunits that assemble into intricate quaternary complexes that ultimately are able to withstand over 50 atm of pressure during genome packaging. The extensive integration between subunits in capsids requires the formation of an intermediate complex, termed a procapsid, from which individual subunits can undergo the necessary refolding and structural rearrangements needed to transition to the more stable capsid. Although various mature capsids have been characterized at atomic resolution, no such procapsid structure is available for a dsDNA virus or bacteriophage. Here we present a procapsid X-ray structure at 3.65 A resolution, termed prohead II, of the lambda-like bacteriophage HK97, the mature capsid structure of which was previously solved to 3.44 A (ref. 2). A comparison of the two largely different capsid forms has unveiled an unprecedented expansion mechanism that describes the transition. Crystallographic and hydrogen/deuterium exchange data presented here demonstrate that the subunit tertiary structures are significantly different between the two states, with twisting and bending motions occurring in both helical and beta-sheet regions. We also identified subunit interactions at each three-fold axis of the capsid that are maintained throughout maturation. The interactions sustain capsid integrity during subunit refolding and provide a fixed hinge from which subunits undergo rotational and translational motions during maturation. Previously published calorimetric data of a closely related bacteriophage, P22, showed that capsid maturation was an exothermic process that resulted in a release of 90 kJ mol(-1) of energy. We propose that the major tertiary changes presented in this study reveal a structural basis for an exothermic maturation process probably present in many dsDNA bacteriophage and possibly viruses such as herpesvirus, which share the HK97 subunit fold.
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Affiliation(s)
- Ilya Gertsman
- Department of Molecular Biology, The Scripps Research Institute, La Jolla, California 92037, USA
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17
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Lee KK, Gan L, Tsuruta H, Moyer C, Conway JF, Duda RL, Hendrix RW, Steven AC, Johnson JE. Virus capsid expansion driven by the capture of mobile surface loops. Structure 2008; 16:1491-502. [PMID: 18940605 DOI: 10.1016/j.str.2008.06.014] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2008] [Revised: 06/10/2008] [Accepted: 06/11/2008] [Indexed: 10/21/2022]
Abstract
The capsids of tailed-DNA bacteriophages first assemble as procapsids, which mature by converting into a new form that is strong enough to contain a densely packed viral chromosome. We demonstrate that the intersubunit crosslinking that occurs during maturation of HK97 capsids actually promotes the structural transformation. Small-angle X-ray scattering and crosslinking assays reveal that a shift in the crosslink pattern accompanies conversion of a semimature particle, Expansion Intermediate-I/II, to a more mature state, Balloon. This transition occurs in a switch-like fashion. We find that crosslink formation shifts the global conformational balance to favor the balloon state. A pseudoatomic model of EI-I/II derived from cryo-EM provides insight into the relationship between crosslink formation and conformational switching.
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Affiliation(s)
- Kelly K Lee
- Department of Molecular Biology, The Scripps Research Institute, La Jolla, CA 92037, USA
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18
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Mutational analysis of a conserved glutamic acid required for self-catalyzed cross-linking of bacteriophage HK97 capsids. J Virol 2008; 83:2088-98. [PMID: 19091865 DOI: 10.1128/jvi.02000-08] [Citation(s) in RCA: 18] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022] Open
Abstract
The capsid of bacteriophage HK97 is stabilized by approximately 400 covalent cross-links between subunits which form without any action by external enzymes or cofactors. Cross-linking only occurs in fully assembled particles after large-scale structural changes bring together side chains from three subunits at each cross-linking site. Isopeptide cross-links form between asparagine and lysine side chains on two subunits. The carboxylate of glutamic acid 363 (E363) from a third subunit is found approximately 2.4 A from the isopeptide bond in the partly hydrophobic pocket that contains the cross-link. It was previously reported without supporting data that changing E363 to alanine abolishes cross-linking, suggesting that E363 plays a role in cross-linking. This alanine mutant and six additional substitutions for E363 were fully characterized and the proheads produced by the mutants were tested for their ability to cross-link under a variety of conditions. Aspartic acid and histidine substitutions supported cross-linking to a significant extent, while alanine, asparagine, glutamine, and tyrosine did not, suggesting that residue 363 acts as a proton acceptor during cross-linking. These results support a chemical mechanism, not yet fully tested, that incorporates this suggestion, as well as features of the structure at the cross-link site. The chemically identical isopeptide bonds recently documented in bacterial pili have a strikingly similar chemical geometry at their cross-linking sites, suggesting a common chemical mechanism with the phage protein, but the completely different structures and folds of the two proteins argues that the phage capsid and bacterial pilus proteins have achieved shared cross-linking chemistry by convergent evolution.
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19
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Hanslip SJ, Zaccai NR, Middelberg AP, Falconer RJ. Intrinsic fluorescence as an analytical probe of virus-like particle assembly and maturation. Biochem Biophys Res Commun 2008; 375:351-5. [DOI: 10.1016/j.bbrc.2008.08.019] [Citation(s) in RCA: 11] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/15/2008] [Accepted: 08/01/2008] [Indexed: 11/28/2022]
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20
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Abstract
In this review, we discuss recent advances in biophysical virology, presenting experimental and theoretical studies on the physical properties of viruses. We focus on the double-stranded (ds) DNA bacteriophages as model systems for all of the dsDNA viruses both prokaryotic and eukaryotic. Recent studies demonstrate that the DNA packaged into a viral capsid is highly pressurized, which provides a force for the first step of passive injection of viral DNA into a bacterial cell. Moreover, specific studies on capsid strength show a strong correlation between genome length, and capsid size and robustness. The implications of these newly appreciated physical properties of a viral particle with respect to the infection process are discussed.
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21
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X-ray solution scattering (SAXS) combined with crystallography and computation: defining accurate macromolecular structures, conformations and assemblies in solution. Q Rev Biophys 2008; 40:191-285. [PMID: 18078545 DOI: 10.1017/s0033583507004635] [Citation(s) in RCA: 845] [Impact Index Per Article: 52.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
Crystallography supplies unparalleled detail on structural information critical for mechanistic analyses; however, it is restricted to describing low energy conformations of macromolecules within crystal lattices. Small angle X-ray scattering (SAXS) offers complementary information about macromolecular folding, unfolding, aggregation, extended conformations, flexibly linked domains, shape, conformation, and assembly state in solution, albeit at the lower resolution range of about 50 A to 10 A resolution, but without the size limitations inherent in NMR and electron microscopy studies. Together these techniques can allow multi-scale modeling to create complete and accurate images of macromolecules for modeling allosteric mechanisms, supramolecular complexes, and dynamic molecular machines acting in diverse processes ranging from eukaryotic DNA replication, recombination and repair to microbial membrane secretion and assembly systems. This review addresses both theoretical and practical concepts, concerns and considerations for using these techniques in conjunction with computational methods to productively combine solution scattering data with high-resolution structures. Detailed aspects of SAXS experimental results are considered with a focus on data interpretation tools suitable to model protein and nucleic acid macromolecular structures, including membrane protein, RNA, DNA, and protein-nucleic acid complexes. The methods discussed provide the basis to examine molecular interactions in solution and to study macromolecular flexibility and conformational changes that have become increasingly relevant for accurate understanding, simulation, and prediction of mechanisms in structural cell biology and nanotechnology.
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Gan L, Speir JA, Conway JF, Lander G, Cheng N, Firek BA, Hendrix RW, Duda RL, Liljas L, Johnson JE. Capsid conformational sampling in HK97 maturation visualized by X-ray crystallography and cryo-EM. Structure 2007; 14:1655-65. [PMID: 17098191 DOI: 10.1016/j.str.2006.09.006] [Citation(s) in RCA: 54] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/07/2006] [Revised: 09/10/2006] [Accepted: 09/12/2006] [Indexed: 11/24/2022]
Abstract
Maturation of the bacteriophage HK97 capsid from a precursor (Prohead II) to the mature state (Head II) involves a 60 A radial expansion. The mature particle is formed by 420 copies of the major capsid protein organized on a T = 7 laevo lattice with each subunit covalently crosslinked to two neighbors. Well-characterized pH 4 expansion intermediates make HK97 valuable for investigating quaternary structural dynamics. Here, we use X-ray crystallography and cryo-EM to demonstrate that in the final transition in maturation (requiring neutral pH), pentons in Expansion Intermediate IV (EI-IV) reversibly sample 14 A translations and 6 degrees rotations relative to a fixed hexon lattice. The limit of this trajectory corresponds to the Head II conformation that is secured at this extent only by the formation of the final class of covalent crosslinks. Mutants that cannot crosslink or EI-IV particles that have been rendered incapable of forming the final crosslink remain in the EI-IV state.
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Affiliation(s)
- Lu Gan
- Department of Molecular Biology, The Scripps Research Institute, La Jolla, California 92037, USA
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Conway JF, Cheng N, Ross PD, Hendrix RW, Duda RL, Steven AC. A thermally induced phase transition in a viral capsid transforms the hexamers, leaving the pentamers unchanged. J Struct Biol 2006; 158:224-32. [PMID: 17188892 PMCID: PMC1978070 DOI: 10.1016/j.jsb.2006.11.006] [Citation(s) in RCA: 33] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/02/2006] [Revised: 09/21/2006] [Accepted: 11/10/2006] [Indexed: 11/19/2022]
Abstract
Scanning calorimetry combined with cryo-electron microscopy affords a powerful approach to investigating hierarchical interactions in multi-protein complexes. Calorimetry can detect the temperatures at which certain interactions are disrupted and cryo-EM can reveal the accompanying structural changes. The procapsid of bacteriophage HK97 (Prohead I) is a 450A-diameter shell composed of 60 hexamers and 12 pentamers of gp5, organized with icosahedral symmetry. Gp5 consists of the N-terminal Delta-domain (11kDa) and gp5* (31 kDa): gp5* forms the contiguous shell from which clusters of Delta-domains extend inwards. At neutral pH, Prohead I exhibits an endothermic transition at 53 degrees C with an enthalpy change of 14 kcal/mole (of gp5 monomer). We show that this transition is reversible. To capture its structural expression, we incubated Prohead I at 60 degrees C followed by rapid freezing and, by cryo-EM, observed a capsid species 10% larger than Prohead I. At 11A resolution, visible changes are confined to the gp5 hexamers. Their Delta-domain clusters have disappeared and are presumably disordered, either by unfolding or dispersal. The gp5* hexamer rings are thinned and flattened as they assume the conformation observed in Expansion Intermediate I, a transition state of the normal, proteolysis-induced, maturation pathway. We infer that, at ambient temperatures, the hexamer Delta-domains restrain their gp5* rings from switching to a lower free energy, EI-I-like, state; above 53 degrees, this restraint is overcome. Pentamers, on the other hand, are more stably anchored and resist this thermal perturbation.
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Affiliation(s)
- James F. Conway
- Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260
| | - Naiqian Cheng
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD 20892
| | - Philip D. Ross
- Laboratory of Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, Bethesda, MD 20892
| | - Roger W. Hendrix
- Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260
| | - Robert L. Duda
- Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260
| | - Alasdair C. Steven
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD 20892
- *Correspondence: Building 50, Room 1517, 50 South Drive MSC 8025, N.I.H., Bethesda, MD 20892, U.S.A., fax 301 443-7651; tel 301 496-0132, E-mail:
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Ross PD, Conway JF, Cheng N, Dierkes L, Firek BA, Hendrix RW, Steven AC, Duda RL. A free energy cascade with locks drives assembly and maturation of bacteriophage HK97 capsid. J Mol Biol 2006; 364:512-25. [PMID: 17007875 PMCID: PMC1941702 DOI: 10.1016/j.jmb.2006.08.048] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/20/2006] [Revised: 08/08/2006] [Accepted: 08/16/2006] [Indexed: 11/26/2022]
Abstract
We investigated the thermodynamic basis of HK97 assembly by scanning calorimetry and cryo-electron microscopy. This pathway involves self-assembly of hexamers and pentamers of the precursor capsid protein gp5 into procapsids; proteolysis of their N-terminal Delta-domains; expansion, a major conformational change; and covalent crosslinking. The thermal denaturation parameters convey the changes in stability at successive steps in assembly, and afford estimates of the corresponding changes in free energy. The procapsid represents a kinetically accessible local minimum of free energy. In maturation, it progresses to lower minima in a cascade punctuated by irreversible processes ("locks"), i.e. proteolysis and crosslinking, that lower kinetic barriers and prevent regression. We infer that Delta-domains not only guide assembly but also restrain the procapsid from premature expansion; their removal by proteolysis is conducive to initiating expansion and to its proceeding to completion. We also analyzed the mutant E219K, whose capsomers reassemble in vitro into procapsids with vacant vertices called "whiffleballs". E219K assemblies all have markedly reduced stability compared to wild-type gp5 (DeltaT(p) approximately -7 degrees C to -10 degrees C; where T(p) is the denaturation temperature). As the mutated residue is buried in the core of gp5, we attribute the observed reduction in stability to steric and electrostatic perturbations of the packing of side-chains in the subunit interior. To explain the whiffleball phenotype, we suggest that these effects propagate to the capsomer periphery in such a way as to differentially affect the stability or solubility of dissociated pentamers, leaving only hexamers to reassemble.
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Affiliation(s)
- Philip D. Ross
- Laboratory of Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, Bethesda, MD 20892
| | - James F. Conway
- Department of Structural Biology, University of Pittsburgh School of Medicine, Pittsburgh, PA 15260
| | - Naiqian Cheng
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD 20892
| | - Lindsay Dierkes
- Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260
| | - Brian A. Firek
- Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260
| | - Roger W. Hendrix
- Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260
| | - Alasdair C. Steven
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD 20892
- *Correspondence: Building 50, Room 1517, 50 South Drive MSC 8025, N.I.H., Bethesda, MD 20892, U.S.A. fax 301 443-7651 tel 301 496-0132 E-mail:
| | - Robert L. Duda
- Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA 15260
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Wikoff WR, Conway JF, Tang J, Lee KK, Gan L, Cheng N, Duda RL, Hendrix RW, Steven AC, Johnson JE. Time-resolved molecular dynamics of bacteriophage HK97 capsid maturation interpreted by electron cryo-microscopy and X-ray crystallography. J Struct Biol 2006; 153:300-6. [PMID: 16427314 DOI: 10.1016/j.jsb.2005.11.009] [Citation(s) in RCA: 51] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/25/2005] [Revised: 11/18/2005] [Accepted: 11/22/2005] [Indexed: 12/01/2022]
Abstract
The bacteriophage HK97 capsid is a molecular machine that exhibits large-scale conformational rearrangements of its 420 identical protein subunits during capsid maturation. Immature empty capsids, termed Prohead II, assemble in vivo in an Escherichia coli expression system. Maturation of these particles may be induced in vitro, converting them into Head II capsids that are indistinguishable in conformation from the capsid of an infectious phage particle. One method of in vitro maturation requires acidification to drive the reaction through two expansion intermediates (EI-I, EI-II) to its penultimate particle state (EI-III), which has 86% more internal volume than Prohead II. Neutralization of EI-III produces the fully mature capsid, Head II. The three expansion intermediates and the acid expansion pathway were characterized by cryo-EM analysis and 3D reconstruction. We now report that, although large-scale structural changes are involved, the electron density maps for these intermediate states are readily interpreted in terms of quasi-atomic models based on subunit structures determined by prior crystallographic analysis of Head II. Progression through the expansion intermediate states primarily represents rigid-body rotations and translations of the subunits, accompanied by refolding of two small regions, the N-terminal arm and a beta-hairpin called the E-loop. Movies made with these pseudo-atomic coordinates and the Head II X-ray coordinates illuminate various aspects of the maturation pathway in the course of which the pattern of inter-subunit interactions is sequentially transformed while the integrity of the capsid is maintained.
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Affiliation(s)
- William R Wikoff
- Department of Molecular Biology, The Scripps Research Institute, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA
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Lee KK, Tsuruta H, Hendrix RW, Duda RL, Johnson JE. Cooperative reorganization of a 420 subunit virus capsid. J Mol Biol 2005; 352:723-35. [PMID: 16095623 DOI: 10.1016/j.jmb.2005.07.024] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/22/2005] [Revised: 07/05/2005] [Accepted: 07/08/2005] [Indexed: 10/25/2022]
Abstract
The complex protein capsids of many viruses exhibit dramatic reorganizations at critical stages in their life-cycle. Here, time-resolved solution X-ray scattering was used to study a dynamic, large-scale conformational maturation of the 420 subunit, 13 MDa, icosahedrally symmetric HK97 bacteriophage capsid. Isoscattering points in the time-resolved scattering patterns and singular value decomposition revealed that the expansion occurs as a cooperative, two-state reaction. The analysis demonstrates that the population shift from Prohead-II to Expansion Intermediate I, EI-I (60 A larger than Prohead-II) occurs in minutes, but does not reveal the time required for individual transitions that occur stochastically. Any intermediate forms that may be traversed during this conversion are unstable and do not constitute an appreciable population of the ensemble of particles. In an energetic landscape view, particles must undergo an energy barrier-crossing event in order to successfully convert from Prohead-II to EI-I. This implies that the particles "hop" over the energy barrier stochastically as they individually attain an expansion-active state. Interestingly, systematic deviations from single-exponential kinetics were observed for the population shift. This may indicate that in undergoing the irreversible conversion from Prohead-II to EI-I, particles are subject to a complex energy landscape that links the initial and final particle forms.
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Affiliation(s)
- Kelly K Lee
- Department of Molecular Biology & Center for Integrative Molecular Biosciences, The Scripps Research Institute, MB-31, 10550 North Torrey Pines Road, La Jolla, CA 92037, USA
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Steven AC, Heymann JB, Cheng N, Trus BL, Conway JF. Virus maturation: dynamics and mechanism of a stabilizing structural transition that leads to infectivity. Curr Opin Struct Biol 2005; 15:227-36. [PMID: 15837183 PMCID: PMC1351302 DOI: 10.1016/j.sbi.2005.03.008] [Citation(s) in RCA: 140] [Impact Index Per Article: 7.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/23/2022]
Abstract
For many viruses, the final stage of assembly involves structural transitions that convert an innocuous precursor particle into an infectious agent. This process -- maturation -- is controlled by proteases that trigger large-scale conformational changes. In this context, protease inhibitor antiviral drugs act by blocking maturation. Recent work has succeeded in determining the folds of representative examples of the five major proteins -- major capsid protein, scaffolding protein, portal, protease and accessory protein -- that are typically involved in capsid assembly. These data provide a framework for detailed mechanistic investigations and elucidation of mutations that affect assembly in various ways. The nature of the conformational change has been elucidated: it entails rigid-body rotations and translations of the arrayed subunits that transfer the interactions between them to different molecular surfaces, accompanied by refolding and redeployment of local motifs. Moreover, it has been possible to visualize maturation at the submolecular level in movies based on time-resolved cryo-electron microscopy.
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Affiliation(s)
- Alasdair C Steven
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, National Institutes of Health, Bethesda, MD 20892, USA.
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Abad-Zapatero C. Homage to Prof. M.G. Replacement: A Celebration of Structural Biology at Purdue University. Structure 2005; 13:845-8. [PMID: 15999422 PMCID: PMC7172766 DOI: 10.1016/j.str.2005.05.003] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
On a glorious spring day in the American Midwest, friends, colleagues, collaborators, and alumni of Prof. M.G. Replacement gathered together at the campus of Purdue University, West Lafayette, Indiana to celebrate 40 years of structural biology and honor the man behind it all: M.G. Rossmann. The date also corresponded approximately to MGR’s 75th birthday. It was a memorable occasion for several reasons. An earlier meeting 10 years ago did also render homage to Michael (New Directions in Protein-Structure Relationships: Symposium in Honor of Professor M.G. Rossmann’s 65th Birthday, Purdue University, October 21, 1995), but on this occasion the symposium was much more encompassing of structural biology and had a more global character. A large number of featured speakers presented and discussed advances in vast areas of structural biology and came from the four corners of the world to share their work with the new generations of structural biologists currently being trained at Purdue University.
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Ross PD, Cheng N, Conway JF, Firek BA, Hendrix RW, Duda RL, Steven AC. Crosslinking renders bacteriophage HK97 capsid maturation irreversible and effects an essential stabilization. EMBO J 2005; 24:1352-63. [PMID: 15775971 PMCID: PMC1142538 DOI: 10.1038/sj.emboj.7600613] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2004] [Accepted: 02/10/2005] [Indexed: 11/09/2022] Open
Abstract
In HK97 capsid maturation, structural change ('expansion') is accompanied by formation of covalent crosslinks, connecting residue K169 in the 'E-loop' of each subunit with N356 on another subunit. We show by complementation experiments with the K169Y mutant, which cannot crosslink, that crosslinking is an essential function. The precursor Prohead-II passes through three expansion intermediate (EI) states en route to the end state, Head-II. We investigated the effects of expansion and crosslinking on stability by differential scanning calorimetry of wild-type and K169Y capsids. After expansion, the denaturation temperature (Tp) of K169Y capsids is slightly reduced, indicating that their thermal stability is not enhanced, but crosslinking effects a major stabilization (deltaTp, +11 degrees C). EI-II is the earliest capsid to form crosslinks. Cryo-electron microscopy shows that for both wild-type and K169Y EI-II, most E-loops are in the 'up' position, 30 A from the nearest N356: thus, crosslinking in EI-II represents capture of mobile E-loops in 'down' positions. At pH 4, most K169Y capsids remain as EI-II, whereas wild-type capsids proceed to EI-III, suggesting that crosslink formation drives maturation by a Brownian ratchet mechanism.
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Affiliation(s)
- Philip D Ross
- Laboratory of Molecular Biology, National Institute of Diabetes, Digestive and Kidney Diseases, Bethesda, MD, USA
| | - Naiqian Cheng
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD, USA
| | - James F Conway
- Institut de Biologie Structurale CEA-CNRS-UJF, Grenoble, France
| | - Brian A Firek
- Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA, USA
| | - Roger W Hendrix
- Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA, USA
| | - Robert L Duda
- Department of Biological Sciences, University of Pittsburgh, Pittsburgh, PA, USA
| | - Alasdair C Steven
- Laboratory of Structural Biology Research, National Institute of Arthritis, Musculoskeletal and Skin Diseases, Bethesda, MD, USA
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