Herpesviruses Flashcards
1
Q
What are the properties of herpesviruses?
A
- Large enveloped viruses
- Double-stranded, linear DNA genome
- All cause lifelong latent infection - most defining feature
- Latent infection can be reactivated
- Make enzymes involved in nucleic acid metabolism - as they infect cells that don’t undergo cell division
- Herpesviruses are ubiquitous
- 3 subfamilies separated by genome sequence and biology
2
Q
Describe alphaherpesvirinae
A
- Short reproductive cycle
- Replicate very quickly in tissue culture
- Cycle 16 hours or less
- Neuronal latency
3
Q
Describe betaherpesvirinae
A
- Long reproductive cycle
- Takes 2 weeks to replicate in tissue culture
- Lymphoid latency - particularly in bone marrow
4
Q
Describe gammaherpesvirinae
A
- Long reproductive cycle
- B cell latency
5
Q
Describe the course of a herpesvirus infection
A
- Acute primary infection - often asymptomatic
- Immune response always leads to latent infection
- Stimulus reactivation can occur leading to acute or subclinical reinfection - infrequent occurrence, virus secretion and new host can be infected
- Herpesviruses are long-term residents of virome
6
Q
What is the most prevalent herpesvirus?
A
HSV-1 - 70% prevalence
7
Q
Describe the herpesvirus structure
A
- Linear DNA is enclosed in an icosahedral capsid
- Enveloped virus with envelope proteins studded in the membrane
- Tegument is a region between the nucleocapsid and the envelope - contains a lot of packaged protein
8
Q
Describe the herpesvirus genome
A
- Large linear dsDNA genomes - 120-230 kb
- Encode > 80 proteins - HIV only has 9
- Express enough proteins to counteract immune response to viral infection
- Uses cellular transcription machinery - uses RNA polymerase II
- Encode their own DNA replication machinery - don’t rely on cellular DNA replication so they can replicate in non cyclic cells
9
Q
Describe the herpesvirus lytic replication cycle
A
- Virus attaches at the plasma membrane and gets into the cytosol
- Delivers its genome into the nucleus
- Genome is transcribed for the immediate-early genes
- mRNA are exported into the cytoplasm to produce immediate-early proteins
- Proteins enter the nucleus to activate the early genes
- Early genes are transcribed and the mRNA exported to the cytosol to make early proteins
- Early proteins enter the nucleus and replicate the viral DNA and activate the late genes
- Late transcripts enter the cytosol and make late proteins
- Some of the structural proteins enter the nucleus and assemble into virus capsids
- Capsids and the genome come together
- Virus assembles into an intact particle
10
Q
Describe the virus attachment and entry
A
- Virus attaches to the cell surface initially by an interaction between a glycoprotein called glycoprotein C (gC) and heparin sulfate proteoglycan (HSPG) on the cell surface - this interaction isn’t essential but if you delete gC it slows entry process
- Virus searches for its specific receptor on the plasma membrane
- Specific receptor is bound by the gD protein on the virus surface
- Other virus proteins initiate fusion between the virus envelope and the plasma membrane - debate in the field whether this fusion occurs at the plasma membrane or in an endocytic vesicle
11
Q
What are the gD receptors for HSV1?
A
- Nectin1 (also known as PVRL1) - probably the major receptor on skin cells, particularly in skin keratinocytes
- HVEM (also known as TNFRSF14) - first gD receptor identified - probably a receptor only on immune cells
- Heparan-sulphate-modified glycoprotein - not sure though
12
Q
How is viral gene expression initiated?
A
- Release of tegument proteins
- HSV capsid is bigger than the nuclear pore
- HSV capsid docks onto the pore and injects its genome into the nucleus
- Genome circularises once in the nucleus
- Tegument protein VP16 enters the nucleus around the same time as the genome
- VP16 activates immediate-early gene expression by promoting transcription factors binding to IE gene promoter targets
13
Q
How does VP16 enter the nucleus and initiate transcription?
A
- VP16 doesn’t have a nuclear localisation signal on it so doesn’t have the ability to go into the nucleus by itself and it’s also a fairly big protein so can’t use nuclear pores
- VP16 binds to HCF-1 protein which shuttles VP16 into the nucleus
- In the nucleus VP16 binds to Oct1 protein
- Oct1 is already bound to the DNA promoter - VP16 doesn’t directly bind to DNA - Oct1 binds to a sequence specific to promoter found only on immediate-early genes
- Trimeric complex of HCF-1, VP16 and Oct1 is able to recruit RNA polymerase II to activate transcription - one of the most powerful transactivators that have been found
14
Q
Describe the DNA replication in the nucleus
A
- Linear viral DNA is circularised during infection
- UL9 binds origins of replication and induces localised melting of duplex DNA
- ICP8 binds single-stranded DNA and promotes UL9-mediated DNA unwinding
- Helicase-primase (UL8/5/52) activity recruits the DNA polymerase holoenzyme (UL30/42) and initiates DNA synthesis via theta replication mode - circular DNA opens up and replication occurs in 2 directions
- In an unknown mechanism, theta replication is converted to rolling circle mode in which head-to-tail concatamers of unit-length viral genomes are produced - produce genomes linked to each other
- Terminase binds to the end of a concatamer and takes it to the portal
- Energy event pushes the DNA into the capsid
- When a full copy of genome has entered the capsid the terminase cuts the concatamer and moves onto another capsid
15
Q
Describe the capsid assembly in the nucleus
A
- DNA replicates in the nucleus
- Icosahedral capsid assembles in nucleus - around 120 nm diameter
- There are 162 capsomeres
- One major capsid protein, VP5, forms pentons and hexons
- BP19c and VP23 form triplexes located between capsomeres
- VP26 associates with hexons - unlike other proteins this protein isn’t absolutely necessary for capsid assembly but it helps assembly
- Uses a dodecameric U6 portal complex to package genomic DNA into the capsid
- Portal = one of the pentamers
