Virus Structure ( Protien )

1. LECTURE 6: Virus Structure(Protien) WaqasNasirChaudhry Viro100: Virology 3 Credit hours NUST Centre of Virology & Immunology

2. 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 bursariaChlorella virus1

3. 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)

4. Non-structural Proteins Their roles include • Enzymes, e.g. protease, reverse transcriptase • Transcription factors • Primersfor nucleic acid replication • Interferencewith the immune response of the host • Cell signaling

5. 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

6. 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

7. 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

8. An icosahedron has five-, three- and two-fold axes of rotational symmetry Vertex 5-fold Triangular face 3-fold Edge 2-fold

9. 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

10. 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

11. 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

12. Enveloped Viruses Lipid membrane in Cytomegalovirus

13. Iridovirus Invertebrate iridescent viruses (IIVs), containing an internal lipid membrane

14. Occlusion bodies • Some viruses provide added protection to the virionswhilst 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 virionin each) • Nucleopolyhedro viruses (many virionsin each)

15. Occlusion bodies • Granuloviruses(single virion in each) • Nucleopolyhedro viruses (many virions in each)

16. Head/Capsid Contractile Sheath Tail Tail Fibers Base Plate Tailed Bacteriophage • Majority of the known phages are tailed phages • dsDNAgenomes • Icosahedral head

17. Cell molecules • Transfer RNA molecules, These are present in the virionsof 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.