VIROLOGY_PARTICLES

Although the icosahedral structure is extremely common among viruses, size differences and slight variations exist between virions. Given an asymmetric subunit on a triangular face of a regular icosahedron, with three subunits per face 60 such subunits can be placed in an equivalent manner. Most virions, because of their size, have more than 60 subunits. These variations have been classified on the basis of the quasi-equivalence principle proposed by Donald Caspar and Aaron Klug.

Construction of an icosahedron is governed by the principles of rotational symmetry of a solid, known as 2–3–5 symmetry and are characterised by  1.) an axis of two-fold rotational symmetry through the centre of each edge- 3 face - and 5 corner.

The triangulation number, T, is defined by: T = f² x P

where f is the number of subdivisions of each side of the triangular face, and f2 is the number of subtriangles on each face; P = h2 + hk + k2, where h and k are any distinct, non-negative integers.  This means that values of T fall into the series 1, 3, 4, 7, 9, 12, 13, 16 and so on.  When P = 1 or 3, a regular icosahedron is formed in contrast with all other values of P give rise to icosahedra of the ‘skew’ class, where the subtriangles making up the icosahedron are not symmetrically arranged with respect to the edge of each face (e.g. in adenovoriruses due point mutations where disrupt symmetry). T = 1 (Microviridae e.g. φX174), T = 3 (many insect, plant, and animal RNA viruses; see below), T = 4 (Togaviridae), and T = 7 (the heads of the tailed bacteriophages such as λ). With larger more complex viruses there is uncertainly - the triangulation number of large and complex Mimivirus particles could have any one of nine values between 972 and 1,200.

Geminivirus particles consist of twinned icosahedra, fused together at one of the pentameric vertices thus  allowing some members of this family to contain a bipartitle genome. Geminivirus particles consist of a fused pair of T = 1 icosahedra joined where one pentamer is absent from each iosahedron.  Geminivirus particles consists of 110 capsid protein subunits and one molecule of ss(+)sense DNA of ~2.7 kb.  Elements of icosahedral symmetry occur frequently as part of larger assemblies of proteins.

Picornavirus particles are icosahedral structures with triangulation number T = 3. Three virus proteins (VP1, 2, and 3) comprise the surface of the particle while VP4, is not exposed on the surface of the virion but is present in each of the 60 repeated units that make up the capsid. Detailed atomic structures of the capsids of a number of different picornaviruses (e.g. poliovirus, foot-and-mouth disease virus)  revealed that the structure of these virus particles is remarkably similar to those of many other genetically unrelated viruses, such as insect viruses of the family Nodaviridae and plant viruses from the comovirus group.  All these virus groups have icosahedral capsids approximately 30 nm diameter with triangulation number T = 3.  The capsid is composed of 60 repeated subassemblies of proteins, each containing three major subunits, VP1, VP2, and VP3.  This means that there are 60 x 3 = 180 surface monomers in the entire picornavirus particle.  All three proteins are based on a similar structure, consisting of 150 to 200 amino acid residues in what has been described as an ‘eight-strand antiparallel β-barrel’.  This subunit structure has been found in all T = 3 icosahedral RNA virus capsids which have been examined so far (e.g. picornaviruses, comoviruses, nepoviruses), possibly showing distant evolutionary relationships between distinct virus families.Picornavirus proteins (and those of other single-stranded positive-sense RNA viruses) are produced by cleavage of a long polyprotein into the final products needed for replication and capsid formation.

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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 asso- ciated with the envelope, usually in small amounts.

Poxivirus particles are large in size, measuring from 200 to 400 nm in length, and are either oval or 'brick-shaped'.They are the most complex of known virions including more than a 1000 virus-encoded proteins. The virion's external surface is ridged in parallel rows frequently present in helical conformations. Replication resultsin the formation of two types of particles, one containing two membranes (extracellular forms)  and the other containing only one  (intracellular forms). These viruses establish two layers of membrane by being wrapped by the endoplasmic reticulum, comprising an alternative process to budding frequently seen in 'simple' enveloped viruses (e.g. retroviruses, influenza) that acquire a single membrane. The bioconcave (dumbell-shapped) core, characterised by two lateral bodies, conists  of the double stranded DNA genome whhich is wound around a tightly compressed nucleoprotein. The whole structure is surrounded by an outter surface layer of lipid and protein nature. Their complexity is reflected in antigenic characteristics where induction of both specific and cross specific-reacting antibodiess takes place, meaning that vaccination for one disease through the usage of another virus ( e.g. immunization against smallpox virus through the use of vaccina virus)

Baculoviruses are natural pathogens of anthroods and naturally-occuring as well as genetically-manipulated are used in studies involving development of pestocides.They contain of 12 to 13 structural proteins. The major nucleocapsid protein VP 39 forms the rod-like nucleocapsid (30 to90 nm in diameter and 200 to 450 nm in length) that includes a double-stranded DNA genome (90-230 kbp) with p6.9 protein.

Some bavulovirus particles exist in two forms, a relatively simple budded form found within the host insect, and a crystalline, protein-occluded form responsible for environmental persistence which enables the virus to persist in soil or on plant materials for extended periods of time until it's ingested by a new host.

Baculoviruses of the occluded form are characterised by an outter protein shell covering the nucleocapsid (entire structure refered to as ' occlusion body;). The two genera of occluded baculoviruses include the Nucleopolyhedrovirus genus, characterised by polyhedral occlusions (1,000 to 15,000 nm in diameter) and which may contain multiple nucleocapsids within the envelope, and the Granulovirus genus, with ellipsoidal occlusions 200 to 500 nm diameter. In addition to the baculoviruses, occluded particles are also produced by insect reoviruses and poxviruses . The occlusion body is alkali liable and dissolves in the pH of the insect midgut thus allowing the release of the nucleocapsid at the appropriate time for replication to ensue. Overexpression of the protein polyhedrin through a strong transcriptional promoter results in the formation of the occlusion body. Cloned foreign genes can be expresed by the polyhedrin promoter thus allowing baculoviruses to be used as expression vectors