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MICR3320 > 6 - Virus Assembly, Release and Maturation > Flashcards

6 - Virus Assembly, Release and Maturation Flashcards

1
Q

Viral assembly

A
  • Refers specifically to the assembly of capsids with genomes and so is universal to all viruses.
  • Enveloped viruses must also acquire a membrane during assembly
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2
Q

Pathway of virus particle assembly and release

A
  • Formation of individual structural units of the protein shell from one or several viral proteins
  • Assembly of the protein shell by appropriate interactions among structural units
  • Selective packaging of the nucleic acid genome and other essential virion compounds
  • Acquisition of an envelope (not all)
  • Release from host cell
  • Maturation of virus particles
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3
Q

Structural units that are often the first assembly intermediates

A

Homo or hetero-oligomers of viral structural proteins from which virus particles are built

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4
Q

Typical sites of assembly in eukaryotic viruses and example for each

A
  • Cytoplasm (bacteriophages)
  • Nucleus (Adenovirus)
  • Nucleus and cellular membranes (herpes and influenza)
  • ER (coronavirus)
  • Plasma membrane (retroviruses)
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5
Q

Importins

A

Cellular proteins that transport cargo proteins from the cytoplasm to the nucleus, aided by nuclear localization sequences (NLS)

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6
Q

Assembly within the nucleus

A
  • Viral proteins destined for the nucleus are synthesized by cytoplasmic polyribosomes, engage with the cytoplasmic face of the nuclear pore complex and are translocated into the nucleus by the protein import machinery of the host cell.
  • E.g. adenoviruses papillomaviruses, and polyomaviruses
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7
Q

Virus assembly from constituent parts

A

Assembly from individual protein molecules (e.g. simian virus) or a polyprotein precursor (e.g. poliovirus) or chaperone assisted assembly (e.g. adenovirus)

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8
Q

Assembly from individual protein molecules

A
  • The assembly reaction is the result of specific interactions among the proteins that form structural units.
  • These assembly reactions are driven in a forward direction by the high concentrations of protein subunits synthesized in
    infected cells
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9
Q

Assembly from a polyprotein precursor

A
  • Illustrated with the poliovirus polyprotein that contains the four proteins that form the heteromeric structural unit.
  • These proteins are synthesized as part of the single polyprotein precursor from which all viral proteins are produced by proteolytic processing including the viral-encoded protease.
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10
Q

Chaperone assisted assembly

A

Some structural units are assembled only with the assistance of viral chaperones, such as the adenoviral L4 100-kDa protein, which is required for formation of the hexon trimer from the protein II monomer.

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11
Q

Chaperones

A
  • Specialised proteins that facilitate the folding of other proteins by preventing improper, nonspecific associations among sticky patches exposed on newly synthesized proteins
  • Abundant in all cells, and some accumulate to concentrations greater than ribosomes.
  • Genomes of several viruses encode proteins with chaperone activity, some with sequences and functions homologous to
    those of cellular proteins
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12
Q

Scaffolds

A
  • Proteins that are needed for parts of the virus to assemble but are not found in the mature virion.
  • Accurate assembly of some large icosahedral protein shells (e.g. adenoviruses and herpesviruses) is mediated by proteins that are not components of mature virus particles
  • E.g. precursor of the herpes simplex virus 1 VP22a protein.
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13
Q

Two strategies of capsids associating with genomes

A
  • Concerted assembly
  • Sequential assembly
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14
Q

Concerted assembly

A
  • The capsomers and genome come together during genome synthesis assembly and there is no separate empty capsid to be filled
  • The nucleocapsid protein encapsidates new genomes while they are being synthesized. - If the virus is enveloped, a later
    assembly step leads to acquisition of a matrix and envelope surrounding the helical nucleocapsid.
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15
Q

Example of concerted assembly

A
  • Rabies virus and Ebola virus have small (−) RNA genomes that are associated with nucleocapsid
    protein N, forming viral ribonuclear proteins (vRNPs)
  • The nucleocapsid N protein is required during genome replication, and the new genomic RNA is encapsidated into RNP during RNA synthesis.
  • Influenza is also a (−) RNA virus that assembles new helical influenza vRNP during RNA synthesis.
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16
Q

Concerted assembly of influenza virus RNPs

A
  • The first stage of influenza assembly is for the PA, PB1, PB2, and NP proteins to form vRNP on new negative strands of RNA through concerted assembly
  • For influenza virus, vRNPs arise from cRNP templates and the genomic vRNPs are packaged into helical structures during genome synthesis
  • vRNPs must be packaged selectively with viral envelope, not cRNPs
  • This is dependent on interactions with PA, PB1, and PB2 and the two RNA molecules
  • These interactions mean vRNP are competent for export out of the nucleus, whereas cRNPs are retained in the nucleus
  • Once exported, sequences near the 5ʹ and 3ʹ ends of the virus genome, which are conserved in all eight genome segments, target the vRNPs for packaging
17
Q

vRNP

A

viral ribonuclear protein

18
Q

cRNP

A

copy ribonuclear protein

19
Q

Sequential assembly

A

During sequential assembly, regular groups of capsomers such as pentamers or hexamers assemble first and then assemble together to make a hollow shell or procapsid.

20
Q

Assembly of HSV1 nucleocapsid

A
  • Requires the scaffolding protein pre-VP22a
  • Self-association of pre-VP22a stimulates binding to VP5, the
    protein that forms structural units of capsid
  • V5 interaction with scaffolding protein guides and regulates VP5 self-assembly
  • Omission of the scaffolding leads to production of partial and deformed nucleocapsid shells
  • Once nucleocapsids have assembled, scaffolding proteins must be discarded, so that viral genomes can be accommodated
21
Q

Virion protease VP24

A
  • Essential for DNA encapsidation
  • This protein is incorporated into the assembling nucleocapsid as a precursor
  • VP24 cleaves the scaffolding protein to remove a short C-terminal sequence that is required for binding to VP5, to disengage scaffolding from structural proteins, once assembly of the nucleocapsid is complete.
  • The VP24 protease also degrades the scaffolding protein so that encapsidation of the genome can begin.
22
Q

Adenovirus L1 proteins

A
  • Form structures that support assembly
  • L1 decreases in concentration as assembly proceeds
23
Q

Viral L3 protease

A

Completes assembly by cleaving precursor proteins, resulting in a mature, infectious virus

24
Q

Localisation or viral proteins to plasma membrane

A
  • Viral envelope glycoproteins are
    cotranslationally translocated into the ER lumen and folded and assembled within that
    compartment.
  • They travel via transport vesicles to and through the Golgi apparatus and then to the
    plasma membrane.
  • The internal proteins of the virus particle and the genome are also directed to plasma membrane sites of assembly
25
Q

Glycosylation of viral proteins

A
  • May alter antigenicity
  • IV HA evolves at the highest rate compared to other influenza viral proteins
  • Presence and acquisition of N-linked glycosylation sites on HA plays a crucial role in immune evasion
  • Number of glycosylation sites on HAs appears to derive from a balance between functionality and immune evasion
26
Q

Two pathways of viruses acquiring envelopes

A

1.Assembly of the capsid and genome occurs first. Acquisition of the envelope occurs next: the assembled capsid interacts with a cellular membrane that also contains virus spike proteins and pinches off (e.g. influenza virus)
2. Formation of internal structures of the virion occur in coordination with the envelope eg. HIV

27
Q

Influenza virus assembly

A
  • IV proteins are synthesized in the cytoplasm and on the endoplasmic reticulum.
  • Viral proteins that will become part of new vRNPs must be trafficked to the nucleus while the viral envelope proteins traffic to the plasma membrane
  • New vRNPs are synthesized in the nucleus and are then exported to the cytoplasm. They travel along the microtubule network at the periphery of the cell, bundling along the way.
  • Eight different vRNPs gather under a patch of plasma membrane containing matrix and spike proteins.
  • The new virion buds away from the surface of the host cell.
28
Q

Final step in IV release

A

Viral NA removes sialic acids from both cellular receptors and from newly synthesized HA and NA on nascent virions, which have been sialylated as part of the glycosylation processes within the host cell

29
Q

Neuraminidase inhibitors

A

Prevent virions from being released from the surface of infected cells

30
Q

HIV1 example of concerted viral assembly

A
  • HIV virion consists of the Env proteins SU and TM, followed by MA matrix protein.
  • Inside of the MA ring, there is a cone-shaped capsid made of the CA protein.
  • The diploid RNA genome closely associated with the NC protein inside the CA.
  • The virion contains the enzymes protease, reverse transcriptase, and integrase, as well as the p6 protein
  • HIV virions also contain a specific tRNA molecule annealed to the RNA genomes
  • Virion also contains the accessory proteins Vif, Vpr, Vpx, and Nef
31
Q

HIV Gag and Gag-Pol polyproteins

A

Give rise to proteins MA, CA, NC, p6, protease, reverse transcriptase, and integrase after proteolysis

32
Q

Maturation

A

Any irreversible process that occurs during or after viral release that takes the assembled virus from a noninfectious to an infectious state

33
Q

HIV-1 protease

A

Cleaves gag and gag-pol polyproteins to produce infectious virus particles with a morphologically distinct core

34
Q

HIV-1 nef

A

Recruits cell-surface CD4 into the endocytic pathway where it is sorted into lysosomes and degraded

35
Q

HIV-1 vpu

A

Mediates translocation of cytoplasmic CD4 in the ER into the cytosol where it is ubiquinated and enters the proteosome to be degraded

36
Q

Release of virus from animal cells

A

Can occur by lysis or budding

37
Q

Adenovirus death protein

A
  • Transmembrane protein that can be found in the nuclear membrane, endoplasmic reticulum, and Golgi apparatus
  • Kills cells when it accumulates during late stages of infection and causes lysis
38
Q

When do enveloped viruses acquire their envelopes

A

Either after the internal parts of the virion have assembled,
(e.g. influenza virus) or during the process of assembling the internal components of the
virion (e.g. HIV)

MICR3320 (14 decks)

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