Virus Structure(Protien)

LECTURE 6:

Viro100: Virology3 Credit hoursNUST Centre of Virology & Immunology

Waqas Nasir Chaudhry

Virus proteins

• Tobacco mosaic virus contains only one protein species

• Parvoviruses contain two to four protein species

• Herpes simplex virus 1 contain 39 proteins• 100 in the virion of the algal virus Paramecium

bursaria Chlorella virus1

Functions of Viral Proteins

• Viral Proteins– Structural Proteins– Non structural Proteins (proteins synthesized by the virus

in an infected cell but they are not virion components)• protection of the virus genome• attachment of the virion to a host cell (for many

viruses)• fusion of the virion envelope to a cell membrane

(for enveloped viruses)

Non-structural Proteins

Their roles include• Enzymes, e.g. protease, reverse transcriptase• Transcription factors• Primers for nucleic acid replication• Interference with the immune response of the

host• Cell signaling

Capsid

• In a virion the virus genome is enclosed in a protein coat, known as a capsid

• Capsid must be stable enough to survive in the extracellular environment

• Alter its conformation so that it can release its genome into the host cell

• Types– Helical symmetry– Icosahedral symmetry

1- Helical Symmetry

• many ssRNA viruses have helical symmetry

• RNA is coiled in the form of a helix and many copies of the same protein species are arranged around the coil– Rigid rod (if strong bonds are present

between the protein molecules in successive turns of the helix)

– Flexible rod (if these bonds are weak)Measles virus nucleocapsid

2- Icosahedron Symmetry

• An icosahedron is an object with• 20 faces, each an equilateral triangle• 12 vertices, each formed where the vertices of

five triangles meet• 30 edges, at each of which the sides of two

triangles meet

• An icosahedron has five-, three- and two-fold axes of rotational symmetry

Vertex

Triangular face

Edge

5-fold

3-fold

2-fold

Capsomeres

• The capsids of some viruses are clearly constructed from discrete structures called Capsomeres

• The capsids of papillomaviruses are constructed from 72 capsomeres, which are all identical

Capsids of some viruses are constructed from two types of capsomere: – Pentons, which are found at the vertices of the

icosahedron – Hexons, which make up the remainder of the

capsid

Structures at capsid vertices

• Some icosahedral viruses have a structure such as a knob, projection or fibre at each of the 12 vertices of the capsid

• Adenovirus virion has a fibre, with a knob attached, at each of the 12 pentons

• Attachment of the virion to its host cell and in delivery of the virus genome into the cell

Enveloped Viruses

Lipid membrane in Cytomegalovirus

IridovirusInvertebrate iridescent viruses (IIVs), containing an

internal lipid membrane

Occlusion bodies

• Some viruses provide added protection to the virions whilst outside their hosts by occluding them in protein crystals

• These occlusion bodies are produced by many of the viruses that infect invertebrates, including most baculoviruses

• Types– Granuloviruses (single virion in each)

– Nucleopolyhedro viruses (many virions in each)

Granuloviruses (single virion in each)

Nucleopolyhedro viruses (many virions in each)

Occlusion bodies

Tailed Bacteriophage

Tail

Tail Fibers

Base Plate

Head/Capsid

Contractile Sheath

• Majority of the known phages are tailed phages• dsDNA genomes• Icosahedral head

Cell molecules

• Transfer RNA molecules, These are present in the virions of retroviruses.

• Proteins, When a virion is assembled some cell protein may be incorporated.

• There are reports of HIV-1 incorporating several cell proteins, including cyclophilin A in association with the capsid and human leukocyte antigens in the envelope.

• Cations, such as Na+,K+,Zn2+ and Mg2+ have also been reported as components of virions.