29. Retroviruses 1 Flashcards
retrovirus replication
- RNA
- DNA
- integration into host genome
- viral protein production
general HIV replication cycle (10 steps)
- attachment
- fusion
- RT
- integration
- mRNA
- translation
- genome recognition
- assembly
- budding
- maturation
what is the role of Gag?
binds genomic RNA for genome recognition for assembly
what happens in acute infection?
1 billion particles are made, but not all are infectious bc of mistakes in splicing, transcription, translation, assembly, etc.
genome of retroviruses
- type
- size
- linear or circular
- segmented?
- haploid or diploid?
- +ssRNA
- 7-12kb
- linear
- non-segmented
- diploid
family of retroviruses?
retroviridae
baltimore class of retroviruses?
group 6
size of virions
8-100nm diameter
hallmark of retroviruses
RT of viral RNA into linear dsDNA which is the reverse of the usual flow of genetic info
2 subfamilies of retroviridae
- orthoretrovirinae
- spumaretrovirinae
what is known about spumaretrovirus?
don’t cause disease
6 genera of orthoretrovirinae and the disease they cause
- alpharetrovirus –> RSV
- betaretrovirus –> mouse mammary tumour virus
- deltaretrovirus –> human T-cell leukemia virus
- epsilonretrovirus –> walleye dermal sarcoma virus
- gammaretrovirus –> murine leukemia virus
- lentivirus –> HIV
Which retroviruses are oncoretroviruses?
alpha, beta, delta, epsilon, gamma
how do lentiviruses cause disease?
killing or inducing loss of function in specific cells and tissues
organization of retroviral genome
3 ORFs
what are the 3 ORFs?
- Gag
- Pol
- Env
what proteins does Gag ORF make?
for structural proteins (matrix, capsid, nucleoprotein)
what is Gag cleaved by?
viral protease made in Pol ORF
what proteins does Pol ORF make?
viral enzymes (RT, integrase, protease)
what proteins does Env ORF make?
surface transmembrane components (viral envelope glycoproteins)
simple vs complex retroviruses
simple: gag, pol, env
complex: gag, pol, env, and auxillary + regulatory proteins
3 viral core proteins
- matrix/p17
- capsid/p24
- nucleocapsid/p7
2 roles of matrix protein
- lines inner surface of viral envelope, linking it to viral core
- transport and binding of Gag to PM during assembly
role of capsid protein
monomers dimerize and form capsid core which surrounds viral genome in mature virus for packaging
role of nucleocapsid protein
monomers associate with 2 copies of viral ssRNA and package them into the viral core
describe stability of capsid core and why this is a good target
weak interactions that can easily disassemble with drugs
what is the capsid structure
200 copies of capsid monomers assembled in hexamers and pentamers, forming a CONE
can molecules go thru capsid?
yes
when do viruses encode an enzyme? (2)
- if there is no equivalent enzyme in the host to hijack
- if it is absolutely necessary for their life cycle
3 virally encoded enzymes:
- RT
- integrase
- protease
describe the RT
RNA-dependent DNA polymerase
what is the issue with RT?
not reliable –> makes errors every time
role of integrase
enables viral genomic material to be integrated within DNA of the cell
role of protease in HIV
cleaves Gag into matrix, capsid, and nucleocapsid proteins
what % of genome is retroviruses?
10%
what are the 2 glycoproteins that HIV Env codes for?
- gp120
- gp41
where is gp120 located?
extracellular
where is gp41 located?
transmembrane
what are the viral spikes made of?
trimer of gp120 and trimer of gp41
describe how envelope glycoproteins determine the tropism of a virus
composition of glycoproteins depends on the retrovirus and determines the tissue and cell tropism of the virus bc they bind to cell-surface receptors
what is required for transcription?
need elements at 5’ region for host transcription factors to bind –> otherwise virus will die
how do retroviruses get multiple transcripts from 1 DNA?
splicing
OVERALL REPLICATION CYCLE (9 steps)
- glycoproteins bind cell receptors
- virus and cell membranes fuse and viral core enters cytoplasm
- in the viral core, RNA is transcribed into DNA by RT
- dsDNA is translocated to nucleus for integration via integrase to form provirus
- provirus is transcribed forming spliced and unspliced mRNA and full-length RNA genomes
- viral mRNAs are translated
- proteins and genomic RNA traffic to PM for assembly
- particle buds from PM, acquiring envelope
- virus matures after budding by proteolytic cleavage of Gag and Gag/Pol polyproteins
3 ways that retroviruses have oncogenic potential
- viral genes encode proteins that drive cell into uncontrolled proliferation
- viral genome integrates adjacent to cellular proto-oncogenes and viral LTR stimulates expression of oncogene (insertional mutagenesis)
- viral gene encodes protein that binds and transactivates host cellular genes to dysregulate cell cycle
example of viral gene encoding proteins that drive uncontrolled proliferation
Src in RSV –> first oncogene discovered
function of Src
tyrosine kinase that phosphorylates targets involved in cell cycle regulation and proliferation to cause uncontrolled cell growth
role of Src in virulence
Src is not required for viral survival but greatly enhances virulence
example of insertional mutagenesis
mouse mammary tumour virus
how does mouse mammary tumour virus work?
produces tumours by inserting genomes into critical sites near/within proto-oncogene –> causing production of active oncogene
example of transactivation of viral and cellular genes
Tax gene in HTLV-1 activates transcription of viral HTLV-1 proteins and human genes involved in cell cycle regulation –> causes adult T-cell leukemia
what are endogenous retroviruses?
genetic elements found ubiquitously in eukaryotic DNA
what do endogenous retroviruses do?
amplify themselves to an RNA intermediate and uses RT + integrase to paste themselves randomly in the genome
2 uses of retroviruses in the biotech industry
- RT-PCR
- Lentiviruses as vectors for targeted gene therapy
- Lentivirues as vectors for Cas9
