Virology Classification of Group V viruses

1. BALTIMORE CLASS V
Kavhumbura Gamuchirai M
Ngara Tanyaradzwa R

2. PRESENTATION OUTLINE
• Background
• Genome features
• Genome replication
• Examples of class v
• Rhabdovirus as a specific example
• Lifecycle of rhabdovirus
• Replication and pathogenicity
• References
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3. Classification and Taxonomy
• the grouping of viruses into an
ordered system that indicates
relationships
Baltimore Classification
• Based on genome composition and
structure
• allows you to:
• 1) deduce the basic steps that must take
place to produce mRNA
• 2) simplifies comprehension of the life
cycle of virus

4. Baltimore Classification

5. Group V
• carry their genetic material in the form
of negative-sense single stranded RNA
• of the order Mononegavirales
• Families: Filoviridae, Paramyxoviridae,
Bornaviridae and Rhabdoviridae
• infecting vertebrates, arthropods,
amoebae, and plants
• Associated with emerging diseases like
Ebola, Marburg

6. Group V
• RNA is not infectious, viruses in this
group encode their own polymerase
(RNA dependent RNA polymerase
[RDRP]
• first a leader RNA is synthesized,
which is followed by sequential
transcription of the genes in the 3’ to 5’
order to yield individual mRNAs by a
stop-start mechanism guided by the
conserved gene-start and gene-end
signals.

7. • The genome encodes for 5 genes in
the following order.
• 3’ –N-P-M-G-L- 5’ N-
• Nucleocapsid protein
• P- Phosphoprotein- cofactor of the
viral polymerase
• M- Inner virion protein/ helps in
budding of the virion.
• G- Glycoprotein that assists in making
virion spikes
• L- Large protein that represents RNA
dependent RNA polymerase and helps

8. Genome features
• Linear non-segmented negative sense
RNA genome
• Organization of genome- 3'-Leader-Virion
core- Surface proteins-Polymerase-Trailer
5'.
• Helical nucleocapsid contains the RNA
dependent RNA polymerase.
• The leader RNA is neither capped nor
polyadenylated and is not functional as
mRNA.

9. Genome Features
• Replication occurs when the
polymerase complex ignores the
transcription stop signals at the 3’ end
of each gene and a full-length positive-
sense anti-genome is synthesized.
• Transcription at the gene-start site is
not perfect, which leads to a gradient
of mRNA abundance that decreases
according to the distance from the 3’
end of the genome.

10. Gradient of mRNA abundance from 3’ end towards 5’ end

11. Replicative Cycle
• As obligate intracellular parasites,
Virus must enter and replicate in living
cells in order to “reproduce”
themselves.
• This “growth cycle” involves specific
attachment of virus, penetration and
uncoating, nucleic acid transcription,
protein synthesis, maturation and
assembly of the virions and their
subsequent release from the cell by
budding or lysis

12. Viral Life Cycle
• Adsorption
• Penetration
• Uncoating and eclipse
• Synthesis of viral nucliec acid and
protein
• Assembly (maturation)
• Release

13. Adsorption
• TEMPERATURE INDEPENDENT
• REQUIRES VIRAL ATTACHMENT
PROTEIN
• CELLULAR RECEPTORS

14. Penetration
ENVELOPED VIRUSES
•Fusion with plasma membrane
•Entry via endosomes
NON-ENVELOPED VIRUSES
•entry directly across plasma membrane

15. Uncoating
• Need to make genome available
• Once uncoating occurs, enter eclipse
phase
• Eclipse phase lasts until first new virus
particle formed

16. Synthesis of nucliec acid and
proteins
• Many strategies
• Nucleic acid may be made in the nucleus
or cytoplasm
• Protein synthesis is always in the
cytoplasm
Assembly and Maturation
• Nucleus
• Cytoplasm
• At membrane

17. Release
• Lysis
• Budding through plasma membrane

18. Examples
Nonsegmented genomes:-
•Rhadboviruses (rabies)
•Paramoxyviruses
Segmented genomes:-
•Orthomyxoviruses (influenza)
•Bunyaviruses
•Arena viruses

19. Rabies virus (Rhabdoviridae)
• Rabies is a zoonotic disease
• Responsible for 55 000 deaths in Africa
and Asia (yearly)
• caused by the bite of a rabid dog/ animal
• Present in the saliva of the infected animal

20. Rabies virus ( Rhabdoviridae)
Bullet shaped structure

21. Virion Properties
• Contain linear, single stranded and
negative sense RNA genome.
• Virions are 45-100nm in diameter, 100-
430nm long.
• virion has a cylindrical nucleocapsid
surrounded by an envelope with large
glycoprotein spikes.
• encode for their own RNA polymerase
(RNA dependent RNA polymerase).

22. Replication
• Replication takes place in cytoplasm
• Virus entry is by receptor mediated
endocytosis.
• Acetylcholine is the receptor
• pH dependent fusion of virus envelope
with endosomal membrane.
• As a result of fusion, the nucleocapsid is
released into the cytoplasm .
• The first step of replication involves mRNA
transcription from genomic RNA using
RDRP.

23. Replication cntd
• For successful replication a large amount
of nucleoprotein (N) and phosphoprotein
(P) should be expressed.
• Switching of transcription to positive sense
antigenome occurs after a threshold
amount of N and P, which are then further
used as a template for synthesis of
negative stranded genomic RNA.

24. Replication C’ntd
• There is a single promoter site at the 3’
end of the viral genome where the
polymerase attaches to the genomic RNA
template and moves along the viral RNA.
• While moving it hits with start – stop
signals at both the ends of the viral genes.
• Due to this only a small fraction undergoes
continuous
• transcription process and hence this
phenomenon is also known as attenuated
transcription

25. Replication C’ntd
• more mRNA is produced towards the
genes that are located at the 3’ end
• *hence producing a gradient of mRNA
in the order of N>P>M>G>L.
• As a result of the mRNA gradient, large
amount of structural protein such as
nucleocapsid protein is produced as
compared to L protein

26. Rabies Virus replication cycle

27. PATHOGENICITY OF THE
VIRUS
• Rabies virus transmitted through the
bite of an infected mammal.
• The virus may enter the peripheral
nervous system directly, or may
replicate in muscle tissue after
entering the host
*remaining at or near the site of
introduction for most of the incubation
period.

28. PATHOGENESIS C’ntd
• Virus may enter the peripheral
nervous system via the
neuromuscular junctions, and moves
rapidly centripetally to the central
nervous system for replication
- symptoms may develop shortly
thereafter.
• The virus then begins to pass
centrifugally to many tissues and
organs, such as the salivary glands.

29. Pathogenesis C’ntd
• In the nervous system the virus is
formed by budding in various
membranes and glands.
• Salivary glands help the virus to bud
on plasma membrane and release it in
very high concentrations through the
saliva
• Death usually occurs due to respiratory
arrest.

30. Progression of rabies virus through neurons
in body:

31. Orthomyxoviridae (segmented)
• contains viruses of single stranded
segmented RNA genome (6-8
segments)
• the most important members of this
family which includes influenza virus A,
B, and C.
• millions of people have been killed.

32. Virion Properties
• Orthomyxovirus virions are
pleomorphic in shape and around 80–
120 nm in diameter
• Contain RNA segments ranging from
8-6
• Genome is single stranded negative
sense RNA of 10 – 14Kb in size
• contain surface glycoproteins over the
lipid envelope: hemagglutinin protein
(H) and neuraminidase protein (N).

33. Schematic representation of an
influenza virus:

34. Bird Flu
• also known as avian influenza (flu
infection in birds).
• The virus caused havoc in the human
population when a bird infecting virus
mutated to infect humans.
• The first case was reported in Hong
Kong in 1997 and that was known as
H5N1 (avian influenza A virus)

35. Viral Replication
• Influenza virus enters the cell after
binding to sialic acid receptor present
on the cell membrane.
• Different cells of the body contain
different sialic acid receptor types
which largely determine the host range
of these viruses. The types of sialic
acid receptor in the epithelium
determines the tropism of human as
well as avian influenza viruses.

36. Viral Replication
• Virus enters the cell by receptor
mediated endocytosis and uncoats
under the low pH condition of
endosome.
• RNA synthesis of influenza virus takes
place in the nucleus of the cell.
Nucleoprotein of influenza virus
contains nuclear localization signals
(NLS) that help in transportation of
ribonucleoprotein complex into the
nucleus.

37. Viral Replication
• Anti sense RNA serves as a template
for the synthesis of the sense strand
• undergo splicing phenomena to
produce two proteins from one gene
such as influenza virus A uses gene
segment 7 to produce M1 and M2
protein.
• Viral protein synthesis occurs in the
cytoplasm
• maturation takes place in endoplasmic
reticulum and Golgi apparatus.

38. Viral replication
• Progeny virus is released by budding
through the plasma membrane.

39. Schematic representation of an influenza
virus replication cycle:

40. Pathogenesis
• enters the host via aerosol it replicates
in the epithelial cells of upper and
lower respiratory tract.
• cause damages in the respiratory
epithelium and alveoli
• The alveolar spaces are filled with
fibrinous exudates and inflammatory
cells.
• Vascular congestions and proliferation
of fibroblast also occurs

41. Pathogenesis
• Varied degree of hepatic damage: as
the virus also replicates in the intestine
of birds.
• necrosis of different visceral organs,
and often succumb to death following
few days of infection