Eukaryotic VirusesVery 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

Contentsnucleic acid (DNA or RNA)a few proteins—to help it establish infectioncapsid—proteinaceous coatlipid 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 arenaked—containing only DNA/RNA and capsidencapsulated –also surrounded by a lipid bilayer derivedfrom a host cell.

Classifications of viruses based on

nucleic acid, capsid and presence or absence of envelope

**

**

RNA virusesServes as mRNA canbe directly translated intoviral proteins!

Must be converted to + RNA first

RETROVIRUSES

CY

TO

PL

AS

MN

UC

LE

US

nucleus cytoplasm

DNA Viruses

nucleus cytoplasm

Capsid—protein coat thatsurrounds DNA or RNA

Shapes of viruses

ICOSAHEDRAL RNA or DNA viruses

HELICAL always an RNA virus

Icosahedral symmetry capsid

Form globular protein frompolypetide chain (3o structure)

Arrange globular proteinsInto equilateral triangle

Place twenty triangles together toform icosahedron

Helical capsid symmetry for RNA viruses

Capsomer—small protein subunits associated with RNA like beads ona stringWhen 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 previouslyinfected

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 lipidbilayer 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 virus1. Make contact with host cell—usually specific2. Bind to a receptor on the cell surface3. Enter the cell via endocytosis or fusion of membranes

4. Uncoat the virus to reveal the nucleic acidsRNA virus –cytoplasmDNA and retroviruses must enter nucleus first

5. Translate mRNA or + stranded RNA that acts like mRNA6. Make proteins required for

structural proteinsproteins responsible for RNA synthesis

7. Exit cell to infect other cells and spread misery.

Life cycle of RNA viruses adsorption and uptake

Naked virus Encapsulated protein

n.b. virus inside ofcell no longer coveredwith lipid

n.b. virus insideof cell now coatedwith lipid, this must be removed

Replication of positive

stranded RNA viruses

+ RNA immediately translatedinto protein required to make -RNA and + RNA (rdRNAP)

The - RNA is replicated viardRNAP to make lots of +RNA

The +RNA is translated to make Coat proteins (capsid)

+RNA GAUCGAUCG-RNA CUAGCUAGC+RNA GAUCGAUCG

original

template

progeny

Replication of negative

stranded RNA viruses

-RNA enters cell withits own vitral RNAP thatconverts – RNA into+ RNA

+RNA translated to makeprogeny – RNA andcapsid

Influenza VirusClass Orthomyxovirus

segmented – stranded RNA virushelical capsid

enveloped

Indications: Fever/chills muscle and joint aches, headachestomach ache and cold-like symptoms

Influenza Virus A infects humans, swine and birds, most likely tocause Flu Pandemics

Influenza Virus B and C only isolated from humans, causes Fluepidemics

Influenza virus—upper respiratory tract cells

M protein (matrix protein) tethers HA and NA to lipid bilayer of virus

NA (neuraminidase) binds to mucinand cleaves the neuramic acid that makesup mucin. Reveals the sialic acid receptor

HA (Hemaglutinase) binds to sialic acidreceptors 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 BUTduring replication of viral RNA small changes are made in the HA and NA genes.

point mutationssmall deletions

This changes the antigenic nature of the HA and NA proteinssuch that our body doesn’t recognize these proteins and MUSTmount 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 aweak immune response. Mild symptoms

Why do we see Flu pandemics that can kill a large number of people?

Antigenic Shift1918—Spanish Flu, killed up to 40 million people worldwide1957—Asian Flu, low mortality1968—Hong Kong Flu, low mortality, avian flu virus the poultrywas 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 controlTreatment DO NOT give aspirin to children, aspirin causes Reye’sSyndrome (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 growthese in chick embryos.Virus isolated, inactivated purified and used to make vaccinesVaccines given to elderly, immuno-compromised andhealth 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 oncogenesthat are inactive unless

a. A carcinogen mutates a region near the geneb. A retrovirus inserts near the proto-oncogene

Acute transforming virus with oncogenes

1. Binds to a receptor2. Taken in by fusionof lipids3. Retroviral RNA convertedto DNA in the cytoplasm viareverse transcriptase4. DNA enters nucleus5. DNA integrates into chromosome via integrase(much like a transposon)6. DNA transcribed intolarge mRNA molecule7. Large protein translated8. Viral proteases cleave large inactive protein into smaller active proteins

Viral oncogeneinserted into chromosme

The product of oncogenes leads to uncontrolled cell growth

Cell surface receptors that bindto mitogens. Mitogens induce Phosphorylation of tyrosine residueson receptors signaling for normalcell growth and division

Similar to EGF, PDGF andinsulin receptor only has MOREtyrosine kinase activity!!!

A retrovirus can integrate near an inactive protooncogene and activate that gene’s

expression

Retroviruscontains promotersthat up-regulate the expression of theproto-oncogene that normallydoes 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 fromsomething else may lead to transcription of the quietintegrated virus.

HIV kills T helper cells

HIV can evade the immune system by travelling from one T-cellto 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 endsrecognized by integrase

reverse transcriptase and envelop genes mutate at a high rate.

HIV VaccinesVaccines to the gp160 protein 1. prevents gp160 from binding to CD4 receptors on T-cells

Caveats1. gp160 changes at a high rate2. HIV can “hide” from antibodies by traveling

from cell to cell without leaving cell

2. Give patients high dosages of CD4 receptors: HIV binds tothe exogenous receptors instead of to T helper cells

HIV treatmentPrevent reverse transcriptase activity or protease activity.

1. NRTI’s (nucleoside reverse transcriptase inhibitors) AZT resembles deoxy-thymidine, therefore as the virus convertsits RNA to DNA it inserts AZT in lieu of dT. DNA elongationaborted.

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 activeproteins.

Usually a combination of all classes of drugs given as virus mutates at a high rate