Transcript Eukaryotic Viruses - Mississippi University for Women

Eukaryotic Viruses
Very small- can only be detected by electron microscopy or by
indirect clinical analysis.
Obligate intracellular parasites: viruses cannot replicate without
the help of a host cell
Contents
nucleic acid (DNA or RNA)
a few proteins—to help it establish infection
capsid—proteinaceous coat
lipid coating taken from host cell (in some cases)
Types of
viruses
--virus
size
compared
to
common
bacteria
Classification of viruses
The nucleic acids they carry (DNA vs. RNA)
The nature of the capsid surrounding the virus (icosahedral vs.
helical)
Whether they are
naked—containing only DNA/RNA and capsid
encapsulated –also surrounded by a lipid bilayer derived
from a host cell.
Classifications
of viruses
based on
nucleic acid,
capsid and
presence or
absence of
envelope
**
**
RNA viruses
CYTOPLASM
Serves as mRNA can
be directly translated into
viral proteins!
Must be converted to + RNA first
nucleus
NUCLEUS
RETROVIRUSES
cytoplasm
DNA Viruses
nucleus
cytoplasm
Capsid—protein coat that
surrounds DNA or RNA
Shapes of viruses
ICOSAHEDRAL  RNA or DNA viruses
HELICAL always an RNA virus
Icosahedral symmetry capsid
Form globular protein from
polypetide chain (3o structure)
Arrange globular proteins
Into equilateral triangle
Place twenty triangles together to
form icosahedron
Helical capsid symmetry for
RNA viruses
Capsomer—small protein subunits associated with RNA like beads on
a string
When RNA forms a helical structure, the capsomer proteins are able to
form one large helical capsid as they interact with each other
Viruses can be naked or encapsulated
This depends on how the virus leaves the host cell it has previously
infected
Naked—If virus reaches critical mass and causes the cell to burst
(similar to P1 bacterial phage we discussed)
Encapsulated—If virus buds out of the cell taking some of the lipid
bilayer from that cell.
When virus buds
from nucleus or cell
membrane it can
take host lipid
bilayer with it—
such virus is
encasulated as
opposed to naked
Typical life cycle of virus
1. Make contact with host cell—usually specific
2. Bind to a receptor on the cell surface
3. Enter the cell via endocytosis or fusion of membranes
4. Uncoat the virus to reveal the nucleic acids
RNA virus –cytoplasm
DNA and retroviruses must enter nucleus first
5. Translate mRNA or + stranded RNA that acts like mRNA
6. Make proteins required for
structural proteins
proteins responsible for RNA synthesis
7. Exit cell to infect other cells and spread misery.
Life cycle of RNA viruses
adsorption and uptake
n.b. virus inside
of cell now coated
with lipid, this must be
removed
n.b. virus inside of
cell no longer covered
with lipid
Naked virus
Encapsulated protein
Replication of
positive
stranded RNA
viruses
+ RNA immediately translated
into protein required to make
-RNA and + RNA (rdRNAP)
The - RNA is replicated via
rdRNAP to make lots of +RNA
The +RNA is translated to make
Coat proteins (capsid)
original
+RNA GAUCGAUCG
template -RNA CUAGCUAGC
progeny +RNA GAUCGAUCG
Replication of
negative
stranded RNA
viruses
-RNA enters cell with
its own vitral RNAP that
converts – RNA into
+ RNA
+RNA translated to make
progeny – RNA and
capsid
Influenza Virus
Class Orthomyxovirus
segmented – stranded RNA virus
helical capsid
enveloped
Indications: Fever/chills muscle and joint aches, headache
stomach ache and cold-like symptoms
Influenza Virus A  infects humans, swine and birds, most likely to
cause Flu Pandemics
Influenza Virus B and C  only isolated from humans, causes Flu
epidemics
Influenza virus—upper respiratory
tract cells
M protein (matrix protein) tethers
HA and NA to lipid bilayer of virus
NA (neuraminidase) binds to mucin
and cleaves the neuramic acid that makes
up mucin. Reveals the sialic acid receptor
HA (Hemaglutinase) binds to sialic acid
receptors of host cell.
Once binding established, virus can fuse
with host cell.
Why do we suffer from the Flu if we have
had it before?
Antigenic Drift
Our body makes antibodies to HA and NA BUT
during replication of viral RNA small changes are made in the
HA and NA genes.
point mutations
small deletions
This changes the antigenic nature of the HA and NA proteins
such that our body doesn’t recognize these proteins and MUST
mount a new immune response
N.B. Flu is usually self limiting  even though the HA and Na has
been changed the change may be small enough that we can mount a
weak immune response. Mild symptoms
Why do we see Flu pandemics that can kill
a large number of people?
Antigenic Shift
1918—Spanish Flu, killed up to 40 million people worldwide
1957—Asian Flu, low mortality
1968—Hong Kong Flu, low mortality, avian flu virus the poultry
was destroyed.
Antigenic Shift leads to a complete change in the NA and/or HA!!!
--2 different influenza viruses attack the same animal
--The RNA of the flu virus is segmented such that different
RNA segments from different sources can be packaged
IN  2 known viruses
OUT 2 new viruses
Treatment and control
Treatment DO NOT give aspirin to children, aspirin causes Reye’s
Syndrome (severe liver and brain pathology)
 Amanatadine or Rimantidine –prevents uncoating of
Influenza Virus A
 Sanamavir (inhaled) oseltamivar (oral) –neuraminidase
inhibitors, neuraminidase cannot break down mucin
Prevention Vaccines.
Scientists choose 3 strains circulating in a population and grow
these in chick embryos.
Virus isolated, inactivated purified and used to make vaccines
Vaccines given to elderly, immuno-compromised and
health care workers.
Retroviruses
Cancer and AIDS
Retroviruses can transform normal cells
into tumor cells by introducing or
activating oncogenes
Oncogenes gene that causes uncontrolled growth of cells
1.carried into cells certain retroviruses (leukemia, sarcomas)
2. Can be present in humans as proto-oncogenes oncogenes
that are inactive unless
a. A carcinogen mutates a region near the gene
b. A retrovirus inserts near the proto-oncogene
Acute transforming virus with
oncogenes
Viral oncogene
inserted into chromosme
1. Binds to a receptor
2. Taken in by fusion
of lipids
3. Retroviral RNA converted
to DNA in the cytoplasm via
reverse transcriptase
4. DNA enters nucleus
5. DNA integrates into
chromosome via integrase
(much like a transposon)
6. DNA transcribed into
large mRNA molecule
7. Large protein translated
8. Viral proteases cleave
large inactive protein into
smaller active proteins
The product of oncogenes leads
to uncontrolled cell growth
Cell surface receptors that bind
to mitogens. Mitogens induce
Phosphorylation of tyrosine residues
on receptors signaling for normal
cell growth and division
Similar to EGF, PDGF and
insulin receptor only has MORE
tyrosine kinase activity!!!
A retrovirus can integrate near an inactive
protooncogene and activate that gene’s
expression
Retrovirus
contains promoters
that up-regulate the
expression of the
proto-oncogene that normally
does not have its own promoter!!
HIV and AIDS
Usually causes acute disease
HIV can integrate into the host chromosome and lie dormant for years
Activation? Stimulation of T-cells by an infection from
something else may lead to transcription of the quiet
integrated virus.
HIV kills T helper cells
HIV can evade the immune system by travelling from one T-cell
to another without leaving the cell
HIV Structure
Gp160 (glycoprotein) binds to CD4 receptors found on
T helper cells
Enters cells with a lot of its own proteins: reverse transcriptase,
integrase and proteases
HIV genes
Sticky ends
recognized by
integrase
reverse transcriptase and envelop genes mutate at a high rate.
HIV Vaccines
Vaccines to the gp160 protein
1. prevents gp160 from binding to CD4 receptors on T-cells
Caveats
1. gp160 changes at a high rate
2. HIV can “hide” from antibodies by traveling
from cell to cell without leaving cell
2. Give patients high dosages of CD4 receptors: HIV binds to
the exogenous receptors instead of to T helper cells
HIV treatment
Prevent reverse transcriptase activity or protease activity.
1. NRTI’s (nucleoside reverse transcriptase inhibitors)
AZT resembles deoxy-thymidine, therefore as the virus converts
its RNA to DNA it inserts AZT in lieu of dT. DNA elongation
aborted.
2. NNRTI’s (non- nucleoside reverse transcriptase inhibitors)
binds to rev. transcriptase thus inactivating it.
3. Protease inhibitors—computer designed peptide analogs that binds to
protease. Large inactive protein not cleaved into smaller active
proteins.
Usually a combination of all classes of drugs given as virus mutates at a high rate