III-VirologyCh28

By :Khaled Alkhodari Sadi Nakhala Mohammed Albelbesi

III Basic Virology

Chapter 281-Virus structure

By: Khaled HANI AlkhodariSupervisor : Dr. Said Alghorrah

Contents :

The flu virus

Introduction

Khaled AlkhodariSize & Shape of VirusesViral Nucleic AcidsViral Capsid & SymmetryViral Proteins Sadi AlnakhalaAtypical Virus-Like Agents Mohammed Albelbesi

Remember

They range from 20 to 300 nm in

diameter =~ largest

protein smallest cell

Size

Shape

-In colloquial terms (spheres, rods, bullets, or bricks).-In reality they are structures of precise

geometric symmetry.-Is determined by the capsid of the virus.

The viral genome is located internally and can be :

-single / double-stranded DNA - - single / double-stranded RNA

Only in Virus

Can be either linear or circular

DNA is always a single molecule RNA = single / several pieces

VIRAL NUCLEIC ACIDS

Both influenza virus and rota virus have a segmented RNA genome

Almost all viruses are haploid.The exception is the retrovirus is diploid.

VIRAL CAPSID & SYMMETRY

Capsid :protein made up from Capsomers : •Consisting of 1/>proteins • Seen in EM as a spherical particle, st with

a central hole• Arrangement=geometric structure of virus

Nucleocapsid = NA + Capsid

VIRAL CAPSID & SYMMETRY

Nucleocapsids have two forms of symmetry:1.Icosahedral: capsomers=20 ▲

symmetric figure (icosahedron) with approximate outline of a sphere.

Enveloped or Naked.

VIRAL CAPSID & SYMMETRY

2. Helical: the capsomers are arranged in a hollow coil that appears rod-shaped.

The helix can be either rigid or flexible.

All helical human viruses are enclosed by an envelopeThere are no naked helical viruses

VIRAL CAPSID & SYMMETRY

Building the virus particle from identical protein subunits has 2 advantages:1- Reduces the need for genetic information.2- Promotes self-assembly (no enzyme or energy is required)

Any Questions?

References

Thank you

PRESENTED BY : SADI YEHIA NKHALAINTRODUCED TO : DR SAID AL-GHORRA

Viral structure

2- proteins

the surface proteins of the virus, whether they are the capsid proteins or the envelope glycoproteins, are the principal antigens against which the host mounts its immune response to viruses.

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The important functions of viral proteins

The capsid proteins protect the genome DNA or RNA from degradation by nucleases.

The proteins on the surface of the virus mediate the attachment of the virus to specific receptors on the host cell surface

Outer viral proteins are also important antigens that induce neutralizing antibody and activate cytotoxic T cells to kill virus-infected cells. These outer viral proteins not only induce antibodies, but are also the target of antibodies (i.e., antibodies bind to these viral proteins and prevent [“neutralize”] the virus from entering the cell and replicating).

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To remember ( bacterial note)

The somatic o-antigen is a polysaccharide on the exterior that differs in each species and frequently differs between strains of a single species .Because the number of permutations of this array is very large, many antigenic types exist , for example, more than 1500 antigenic types have been identified for Salmonella.

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SEROTYPES ! ( ANTIGENIC DETERMINANTS)

The term “serotype” is used to describe a subcategory of a virus based on its surface antigens. For example, measles virus has one serotype, polioviruses have three serotypes , and rhinoviruses have over 100 serotypes.

This is because all measles viruses have only one antigenic determinant on its surface protein that induces neutralizing antibody capable of preventing infection. In contrast, polioviruses have three different antigenic determinants on its surface proteins ,i.e., poliovirus type 1 has one kind of antigenic determinant , poliovirus type 2 has a different antigenic determinant, and poliovirus type 3 has a different antigenic determinant from types 1 and 2

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Medical implications related to serotypes

person can be immune (have antibodies) to poliovirus type 1 and still get the disease ,poliomyelitis caused by poliovirus types 2 or 3.

The other implication is the polio vaccine must contain all three serotypes in order to be completely protective.

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Internal viral proteins ( types and variablility)

Some of the internal viral proteins are structural (e.g. , the capsid proteins of the enveloped viruses), whereas others are enzymes (e.g., the polymerases that synthesize the viral mRNA).

The internal viral proteins vary depending on the virus. Some viruses have a DNA or RNA polymerase attached to the genome; others do not.

If a virus has an envelope, then a matrix protein that mediates the interaction between the capsid proteins and the envelope proteins is present.

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Summary :

Internal proteins may be :

1-capsid proteins ( in an enveloped viruses )

2-enzymes ( polymerase )

3-matrix proteins ( in an enveloped viruses )

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Produced Proteins ( Superantigens )

Some viruses produce proteins that act as “superantigens” similar in their action to the superantigens produced by bacteria, such as the toxic shock syndrome toxin of Staphylococcus aureus .

Viruses known to produce superantigens include: 1- Two members of the herpes virus family, namely, Epstein-Barr virus and cytomegalovirus, 2- The retrovirus( mouse mammary tumor virus.)

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Regulatory Proteins: Some viruses contain regulatory

proteins in a structure called the tegument, which is located between the nucleocapsid and the envelope. These regulatory proteins include transcription and translation factors that control either viral or cellular processes. Members of the herpesvirus family, such as herpes simplex virus and cytomegalovirus, have a prominent, well-characterized tegument.

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Viral Envelope The envelope is a lipoprotein

membrane composed of lipid derived from the host cell membrane and protein that is virus-specific.

Furthermore, there are frequently glycoproteins in the form of spike-like projections on the surface, which attach to host cell receptors during the entry of the virus into the cell.

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Viral Envelope The envelope of most viruses is derived from

the cell’s outer membrane( cytoplasmic membrane ) , with the notable exception of herpesviruses that derive their envelope from the cell’s nuclear membrane.

In general, the presence of an envelope confers instability on the virus. Enveloped viruses are more sensitive to heat , drying , detergents, and lipid solvents such as alcohol and ether than are nonenveloped viruses, which are composed only of nucleic acid and capsid proteins.

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Viral Envelope

An interesting clinical correlate of this observation is that virtually all viruses that are transmitted by the fecal– oral route (those that have to survive in the environment) do not have an envelope, that is, they are naked nucleocapsid viruses. These include viruses such as hepatitis A virus, poliovirus, Coxsackie virus, echovirus, Norwalk virus, and rotavirus.

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Viral Envelope

In contrast, enveloped viruses are most often transmitted by direct contact, such as by blood or by sexual transmission. Examples of these include human immunodeficiency virus, herpes simplex virus type 2, and hepatitis B and C viruses. Other enveloped viruses are transmitted directly by insect bite (e.g., yellow fever virus and West Nile virus) or by animal bite (e.g., rabies virus).

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Viral Envelope

Many other enveloped viruses are transmitted from person to person in respiratory aerosol droplets, such as influenza virus, measles virus, rubella virus, respiratory syncytial virus, and varicella-zoster virus. If the droplets do not infect directly, they can dry out in the environment, and these enveloped viruses are rapidly inactivated. .

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Viral Envelope

Note that rhinoviruses, which are transmitted by respiratory droplets, are naked nucleocapsid viruses and can survive in the environment for significant periods. They therefore can also be transmitted by hands that make contact with the virus on contaminated surfaces

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Summary1-Nonenveloped visrus : Feco-oral route / Respiratory droplets

2-Enveloped viruses : Direct contact / Bites / Respiratory droplets

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3-Structure of Viruses(Atypical virus-like agents )

Prepared by : Mohammed Elbelbessi

supervised by : Dr- Said Elghora

Characteristics of typical viruses

1- viruses range in size from (20 – 300 nm) 20~ the size of large proteins

300 ~ the size of the smallest cells

2- viruses contain either DNA or RNA , but not both

3- All viruses have a protein coat called a capsid

4- viral proteins serve several important function

Atypical viruses

There are four exceptions to the typical viruses .

1- Defective viruses

2- Pseudovirions

3- Viroids

4- Prions

Defective viruses

Defective viruses are composed of nucleic acid and protein But , they can't replicate without a “ Helper virus “ .

Fgf

During growth of most human viruses , more defective viruses are produced than infectious viruses Ratio ( 100 :1 )

Defective viruses

*- The growth of defective and non-defective viruses replicate simultaneously

Because defective particles formation can interfere with the growth of the infectious particles . It has been hypothesized that the defective viruses may aid in recovery from an infection by limiting the ability of infectious particles to grow

e.g , ( production of protecting viruses )

Pseudovirions contain host cell DNA instead of viral DNA

*- when the cell DNA is fragmented during cell injury , pieces of DNA is incorporated within the capsid protein

*- Pseudovirions unlike defective viruses can infect cells , but they don’t replicate

Pseudovirions

Viroids

• Consist only of single molecule of circular RNA Without protein coat or envelope . There is extensive homology

between bases in viroid RNA And doesn’t code for any protein

*- viroids replicate , but the mechanism is poorly understood.*- cause several plant disease , but not human ones

Prions

Prions are infectious particles that are composed only of proteins ( No detectable nucleic acid ) .

*- they cause several diseases such as Creutzfeldt-Jakob disease in humans and scrapie in animals.

Creutzfeldt-Jakob = mad cow disease = BSE

Prions

Prions

*- Prions are more resistant to inactivation by ultraviolet light and heat than viruses .

*- prions composed of a single glycoprotein with a molecular weight (27,000 – 30,000 ) .

*- prion proteins are encoded by single circular gene And this gene is found in equal numbers in infected and uninfected animals .

Prions

To differentiate between proteins found in infected and uninfected cells. Post- transitional modification of the prion proteins are hypothesized .

These modification include change in the structure of Proteins from alpha – helical form to beta- pleated sheet . Change occurs ( affected with scrapie )

Functions of normal prion protein

1-one of the signal transduction proteins in neurons2- copper- binding protein 3- protease-sensitive in normal cells , and protease- resistant in infected

cells 4- No immune response formed against this protein and so, No inflammatory response .

Prions

Prions proteins in infected brain tissue form rod-shaped particles that are morphologically and histochemically indistinguishable from amyloid

Amyloid : substance formed in brain tissue after various central nervous system diseases

amyloidosis also occur in other organs such as bone and joints

Prions