Overview of virus life cycle

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1 Overview of virus life cycle cell recognition and internalization release from cells progeny virus assembly membrane breaching nucleus capsid disassembly and genome release replication and translation

2 Virus assembly and maturation Viral proteins and nucleic acid are generated separately, sometimes in separate compartments Must be brought together, in correct manner, to form infectious particles

3 Modes of assembly Self-assembly (like crystallization), components spontaneously combine to form particles that represent the minimum energy state Particles assemble from precursor proteins, which are then modified to form the virion Viral genome encodes morphogenetic factors which are required for assembly but are not part of the particles

4 Self-assembly Example - Tobacco Mosaic Virus (TMV). First macromolecular structure to self-assemble in vitro One long, helically wound molecule of RNA 2130 molecules of coat protein also arranged helically (A protein) A Klug, Phil. Trans. R. Soc. Lond. B. (1999)

5 Intermediates formed by TMV at different ionic and ph conditions A Klug, Phil. Trans. R. Soc. Lond. B. (1999)

6 Self-assembly of TMV The 20S structure binds to RNA through a stemloop hairpin The hairpin inserts in the hole between the disk A Klug, Phil. Trans. R. Soc. Lond. B. (1999)

7 Assembly from precursor proteins Example - poliovirus Poliovirus capsid = 60 x [VP1+VP2+VP3+VP4]

8 Assembly of poliovirus VP1 + VP0 + VP3 (5S) pentamers (14S) 12 pentamer aggregate (73S) addition of genomic RNA (125S) Morphogenetic cleavage VP0 VP2 + VP4 Converts particle to 150S

9 Directed assembly (with morphogenetic factors) Head-tail structures - bacteriophage T4 bacteriophage P22 HK97 bacteriophage lambda

10 Directed assembly Complex assembly process Head, tail, tail fibers all synthesized separately A number of gene products involved (structural and non-structural) Gene products identified through electron microscopic studies of conditional lethal mutants

11 Identification of genes involved in T4 assembly Genes Phenotype 20,21,22,23,24,31,40 No heads or aberrant heads 2,4,16,17, 49,50,64,65 Inactive heads 5,6,7,8,10,11,25,26,27,28, 29,51,53 No base plates 3,15,18,19 Incomplete tails 34,35,36,37,38,57 No tail fibers 13,14 Normal head, tail, no mature particles

12 In vitro complementation studies Gene 34 mutant Gene 23 mutant Purification of components Infectious phage

13 Determining a sequence of events Mutant gene Phenotype 23 No head structure 31 Lumps 22 Multilayered polyheads 20, 40 Single layered polyheads 21, 24 Tau particles /40 21/24

14 Head assembly pathway Cleavage of P23 to P23* Gene 16 and 17 Gene 49 Prohead I 400S: No DNA Prohead II 350S: No DNA Prohead III 550S: 50% DNA Mature head 1100S: 100% DNA

15 Pathway of tail assembly Protein 19 (core) Protein 3 (sheath stabilizer) Protein 18 (sheath) Protein 15 (head-tail connector)

16 Pathway of T4 virus assembly + Tail fibers Product of gene 63

17 Tail only attaches to one Vertex DNA protrusion acts as signal Lander et al., Science, 2006

18 Involvement of scaffolding proteins Proteins that help in virion formation Not found in mature virions Promote assembly by 1) Reducing coat protein concentration required for assembly 2) Guides proper formation of the shell

19 Involvement of scaffolding proteins T4 gp22 P22 gp8 HK97 Delta domain of coat protein Herpesviruses UL26.5 Removed from prohead at the time of DNA encapsidation

20 Delta domain of HK97 Johnson JE, Curr Opin Struct Biol, 2010

21 Assembly of bacteriophage P copies of gp5 250 copies of gp8 Lander et al., Science, 2006

22 DNA packaging signal Length of genome 41.7 kbp Packaged genome 43.5 kbp Head-full packaging DNA packaging triggers expansion Lander et al., Science, 2006

23 Maturation Proteolytic cleavage event/cross-linking/binding to accessory proteins Converts procapsid to capsid Accompanied by change in head diameter, thickness Metastable intermediate states store energy Required during infection

24 Maturation of HK97 Johnson JE, Curr Opin Struct Biol, 2010

25 Assembly of enveloped viruses

26 Assembly of enveloped viruses I. Formation of the nucleoprotein complex II. Synthesis of viral glycoproteins and their incorporation in patches of cellular membrane III. Interaction of matrix proteins with the cytoplasmic tails of viral glycoproteins IV. Pinching of cellular membranes around viral nucleoprotein Cellular membrane proteins excluded

27 Assembly sites for enveloped viruses Apical membrane - Orthomyxovirus Paramyxovirus Basolateral membrane - Alphavirus Rhabdovirus HIV ER membrane - Coronavirus Flavivirus Golgi complex - Bunyavirus Coronavirus Nuclear envelope - Herpesvirus

28 Assembly of enveloped viruses Budding = membrane fission energy source? Late (L) domains identified in viral proteins Required for virus particle budding L domains interact with cellular proteins involved in vacuolarization pathways (VPS, ESCRT) Constitutes pushing force for budding Viral proteins form microdomains on plasma membrane Viral budding zones budzones Induce membrane curvature?

29 Assembly of enveloped viruses Chen and Lamb, Virology, 2008

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