Biotech 426 Virus Structure & Classification PDF

(Biotech 426: Molecular Virology and Vaccine Production) Virus Classification & properties [Structural properties: Capsid & Envelope] Virus Classification  HolmesClassification Group I: Phaginae Group II: Phytophaginae Gro...

(Biotech 426: Molecular Virology and Vaccine Production) Virus Classification & properties [Structural properties: Capsid & Envelope] Virus Classification  HolmesClassification Group I: Phaginae Group II: Phytophaginae Group III: Zoophaginae LHT system Classification First taxonomic system Lwoff, Horne, and Tournier (the LHT system): first published in 1962 and finalized in 1966 Phylum Vira Subphyla: Deoxyvira Ribovira Class Deoxybinala Ribocubica Deoxyhelica Ribohelica Deoxycubica  ICTV Classification It is the structure based classification of viruses proposed by International Committee on Taxonomy of Viruses (ICTV). In 1966, International Committee on Nomenclature of Viruses (ICNV) was established and renamed as ICTV in 1974. ICTV is assigned with the task of a universal taxonomic scheme for all the viruses infecting animals, plants, fungi, bacteria etc. Classes were further divided into orders (“…virales”) suborders (“…viridales”) families (“…viridae”) subfamilies (“…virinae”) genera (“…virus”) subgenera (“…virus”)  Baltimore Classification Baltimore classification of viruses is based on the messenger RNA synthesis. There are seven groups under which viruses are classified. These are listed below. Group-I: Double-stranded DNA viruses e.g. Herperviridae, Iridoviridae, Poxviridae, Adenoviridae, Asfaviridae, Papillomaviridae Group II: Single-stranded DNA viruses e.g. Parvoviridae, Circoviridae Group III: Double-stranded RNA viruses e.g. Birnaviridae, Reoviridae Group IV: Single-stranded RNA viruses (Positive sense) e.g. Arteriviridae, Coronaviridae, Picornaviridae, Flaviviridae, Togaviridae, Astroviridae  Baltimore Classification Contd.. Group V: Single-stranded RNA viruses (Negative sense) e.g. Arenaviridae, Bunyaviridae, Orthomyxoviridae, Paramyxoviridae, Bornaviridae, Rhabdoviridae, Filoviridae Group VI: Single-stranded RNA viruses with a DNA intermediate in their life cycle e.g. Retroviridae Group VII: Double-stranded DNA viruses with an RNA intermediate in their life cycle e.g. Hepadnaviridae  Size: varies from 18-20 nm (Parvovirus) to 130-375 nm (Poxvirus); even some filamentous Filovirus (upto 14000nm) Protein coat of the virus: Capsid Capsid with nucleic acid genome: Nucleocapsid Structural subunit: Capsomers Envelope: Outside the capsid some viruses have an envelope made of Lipoprotein Peplomers: The envelope of some viruses contain spikes called as peplomers Virions: The complete viruses with the structural components Virus Symmetry: shape of the capsid/nucleocapsid after arrangement of capsomers is called symmetry of the viruses  The structural units or capsomers on the faces and edges of adenovirus virions, for example, bond to six neighbouring capsomers and are called hexons; those at the vertices bond to five neighbours and are called pentons In the virions of some viruses, both hexons and pentons are composed of the same polypeptide(s), whereas in those of other viruses they are formed from different polypeptides. Helical viruses are defined by their amplitude (diameter) and pitch (the distance covered by each complete turn of the helix). Tobacco mosaic virus (TMV) has a capsid consisting of many molecules of a single-coat protein arranged in a constant relationship, forming a helix with a pitch of 2.28 Å Rhabdovirus particles, such as those of vesicular stomatitis virus, have an inner helical nucleocapsid surrounded by an outer lipid envelope and its associated glycoproteins More complex (higher order) icosahedra can be defined by the triangulation number of the structure, T = f2 ¥ P. Regular icosahedra have faces consisting of equilateral triangles and are formed when the value of P is 1 or 3. All other values of P give rise to more complex structures with either a left-hand or right-hand skew  Caspar-Klug principle Virion minimizes free energy by organizing capsomers quasi-equivalently Icosahedra with triangulation numbers of 1, 3, and 4 Triangulation number T=a2+ab+b2  Enveloped virus particles are formed by budding through a host cell membrane, during which the particle becomes coated with a lipid bilayer derived from the cell membrane. For some viruses, assembly of the structure of the particle and budding occur simultaneously, whereas in others a preformed core pushes out through the membrane RELEASE  Several classes of proteins are associated with virus envelopes. Matrix proteins link the envelope to the core of the particle. Virus-encoded glycoproteins inserted into the envelope serve several functions. External glycoproteins are responsible for receptor recognition and binding, while transmembrane proteins act as transport channels across the envelope. Host-cell-derived proteins are also sometimes found to be associated with the envelope, usually in small amounts (a) Simple enveloped viruses acquire a single-layer envelope by budding at the cell surface or into intracellular compartments. (b) Complex enveloped viruses acquire multiple membrane layer by being wrapped by the encoplasmic reticulum or other cellular membranes. Poxvirus particles are the most complex virions known and contain more than 100 virus-encoded proteins, arranged in a variety of internal and external structures Thank You