Lec 12: Retroviruses Flashcards
Retrovirus classes =
Class VI & VII
The discovery of retroviruses challenged
the traditional central dogma.
DNA –> RNA –> Protein
Reverse transcription happens not only in…
but also in…
…RNA viruses,
…DNA viruses!
HIVs are
retroviruses
Retroviruses may cause
cancer
Retreovirus infection has been found in
all classes of vertebrates, including fish, amphibians, birds and mammals.
Endogenous retroviruses:
as normal genetic elements in their chromosomal DNA
Exogenous retroviruses:
horizontally-transmitted infectious RNA-containing viruses which are transmitted from human-to-human (e.g. HIV and human T cell leukemia virus, HTLV)
In general, endogenous human retroviruses are
not pathogenic and many of them are not complete viruses
The human genome contains between 100–1,000 copies of…
However, some…
…endogenous viruses and many of them have become pseudogenes or have various defects
…are complete viruses and the genes of some are transcribed and make virus-encoded proteins
In a latent herpesvirus infection…
multiple genome is switched off during latency, but a few regions are transcribed and a few RNAs are synthesized; some viruses also synthesize a few proteins.
No virus proteins are required to maintain
latency in cells that do not divide, so none are produced in neurons latently infected with HSV-1.
Virus RNAs synthesized during latency are known as
latency associated transcripts (LATs).
The LATs undergo…
the greater the degree to which the host is…
…splicing, and at least one of them plays a role in inhibiting apoptosis, thereby ensuring the survival of the neurone with its latent HSV-1 infection.
…immunocompromised, the greater the likelihood of reactivation.
latency associated transcripts (LATs) =
Virus RNAs synthesized during latency
Retroviruses exist in 2 forms:
- ) RNA-containing virions which bud from a producing cell and can infect another cell,
- ) DNA proviruses which may be active or silent.
Proviruses exist…
…naturally in most vertebrates, as well as some non-vertebrates, and are present in the germ line as “normal” genes.
Retrovirus virion contains…
present as…
…2 copies of the RNA genome
(t.f. virion = diploid)
…a dimer, formed by base pairing between complementary sequences
tRNAs from ____ ____ function as _______.
host cells
primers
cool retrovirus complex
kissing-loop complex
A number of protein species are…
The most abundant protein is…
Other proteins,…
…associated with the RNA
…the nucleocapsid (NC) protein, which coats the RNA.
…present in much smaller amounts, have enzyme activities.
The genes encoding the retrovirus proteins are organized in…
3 major regions of the genome:
- ) gag (group-specific antigen) = internal structural proteins.
- ) pol (polymerase) = enzymes
- ) env (envelope) = envelope proteins.
Retrovirus Attachment and entry:
mostly by…
some by…
mostly by = fusion
some by = endocytosis
Retrovirus Attachment and entry 2 steps:
- ) The virion contents released into the cytoplasm loses some proteins.
- ) The contents undergo modification to form a reverse transcription complex.
Reverse transcription takes place
within the reverse transcription complex.
(Reverse transcription) primer for (−) DNA synthesis = primer for (+) DNA synthesis =
(-) = tRNA bound to the genome (+) = a polypurine tract (PPT) in the virus genome
polypurine tract (PPT) =
a sequence made up entirely, or almost entirely, of purine residues (in the virus genome.)
Purines =
Pyrimidines =
Purines = A / G Pyrimidines = C / U / T
PBS: PPT: R: U3: U5:
PBS: primer binding site. PPT: polypurine tract R: repeat sequence. U3: unique sequence at 3' end of genome. U5: unique sequence at 5' end of genome.
Reverse Transcription Steps: (8)
- ) A copy of the virus genome with a tRNA bound at the PBS.
- ) Reverse transcriptase begins (−) DNA synthesis @ 3’ end of the tRNA.
- ) The RNase H digests the RNA from the RNA-DNA duplex. The (−) DNA attaches at the 3’ end of either the same strand or the 2nd copy of the genome.
- ) Elongation of the (−) DNA continues, while the RNase H degrades the template RNA from the 3’ end as far as the PPT.
- ) Synthesis of (+) DNA begins.
- ) Remaining RNA is degraded.
- ) The (+) DNA detaches from the 5 end of the (−) DNA template and attaches at the 3 end.
- ) Synthesis of both DNA strands is completed.
(Reverse Transcription)
During synthesis of the two DNA strands…
each detaches from its template and re-attaches at the other end of the template through base pairing.
Integration of the provirus happens when…
For most retroviruses…
…the provirus is transported to the nucleus as a pre-integration complex.
…this can occur only if the cell goes into mitosis.
What is likely necessary for the pre-integration complex to enter the nucleus?
What does this mean?
HIV, however, can…
- mitosis-induced breakdown of the nuclear membranes
- there can be a productive infection only in dividing cells.
…productively infect resting cells, as the pre-integration complexes of these viruses are able to enter intact nuclei.
One viral protein still associated with the provirus is called
integrase
integrase function =
therefore making it…
= cuts the DNA of a cell chromosome and seals the provirus into the gap
…a multifunctional enzyme!
The integrated provirus genes may…
or there may be…
…be expressed immediately = productive infection
…little or no expression = latent infection.
If a latently infected cell divides
the provirus is copied along with the cell genome and each of the daughter cells has a copy of the provirus.
(Retrovirus Transcription and genome replication)
What is synthesized?
and how will it function?
Genome-length RNAs are synthesized.
Some will function as mRNAs (green) while some will become the genomes of progeny virions (blue) (these RNAs are identical)
(Assembly and release of virions)
Some retroviruses form…
But most retroviruses…
…immature particles in the cytoplasm that are then transported to the plasma membrane
…assemble their components on the inner surface of the plasma membrane
most retroviruses assemble their components on
the inner surface of the plasma membrane
Two copies of the genome associate with
cell tRNAs and with Gag & Gag–Pol proteins.
The immature retrovirus virion acquires its envelope by
budding from the cell surface
Retrovirus classification depends on
the complexity of their genomes
The simple retroviruses have only…
or in some cases…
…the 3 standard retrovirus genes (gag, pol, env), or in some cases
…one additional gene, called an oncogene because its expression might result in its host cell developing into a tumour cell.
oncogene =
gene who’s expression might result in its host cell developing into a tumour cell.
An example of an oncogene is
src in the genome of Rous sarcoma virus, which infects chickens.
The complex retroviruses have
additional genes, the products of which have a variety of functions in the replication cycle (ex: HIV)
Some genetically modified retroviruses are used as
gene vectors
to introduce genes into a variety of cell types, where the genes are expressed at high levels after integration into the cell genomes.
(Retroviruses as gene vectors)
Purpose of gene delivery: (3)
- ) Study the function of specific genes in lab
- ) Treat genetic disorders
- ) Treat cancers.
murine leukaemia virus =
Lentiviral vectors have also been developed, they can deliver genes into non-dividing cells and tissues.
retroviral vectors can successfully treat
Patients with the genetic disorder X-linked severe combined immunodeficiency (X-SCID, an immunodeficiency disorder in which the body produces very little T cells and NK cells)
The genomes of vertebrate animals contain
retroviral sequences
The genomes of most of these endogenous retroviruses (ERVs) are
defective
Human genome has almost 100K
human ERV (HERV) sequences.
Some ERVs are closely related to
normal retroviruses (exogenous retroviruses).
ERVs =
endogenous retrovirusew
It is highly likely that ERVs originated as a result of…
If one of these cells with an integrated provirus survived to be involved in the reproductive process, then…
…exogenous retroviruses infecting germ line cells (sperm and/or egg).
…each cell in the body of the offspring would contain a copy of the provirus.
Most ERVs are defective, so they do not
normally replicate
Missing ERV functions may be supplied by
another ERV or an exogenous retrovirus, so some ERVs can replicate.
Some ERVs are not
defective;
they have an intact genome and can initiate a productive infection.
Some ERVs do not replicate in…
Ex: ?
…cells of the species in which they occur, but are able to replicate in the cells of other species.
Ex: some mouse ERVs and some pig ERVs can replicate in human cells – RISK in cross species organ transplantation.
ERV Concerns:
there may be a risk of transmitting retroviruses from pigs into humans if pigs are used as sources of cells, tissues and organs because of shortages of their human counterparts for transplant purposes.
