Editing Capsid (section)

===Prolate===
[[Image:PhageExterior.svg|thumb|left|The prolate structure of a typical head on a [[bacteriophage]]]]
An elongated icosahedron is a common shape for the heads of bacteriophages. Such a structure is composed of a cylinder with a cap at either end. The cylinder is composed of 10 elongated triangular faces. The Q number (or T<sub>mid</sub>), which can be any positive integer,<ref>{{cite journal | vauthors = Luque A, Reguera D | title = The structure of elongated viral capsids | journal = Biophysical Journal | volume = 98 | issue = 12 | pages = 2993–3003 | date = June 2010 | pmid = 20550912 | pmc = 2884239 | doi = 10.1016/j.bpj.2010.02.051 | bibcode = 2010BpJ....98.2993L }}</ref> specifies the number of triangles, composed of asymmetric subunits, that make up the 10 triangles of the cylinder. The caps are classified by the T (or T<sub>end</sub>) number.<ref>{{cite book | vauthors = Casjens S |title=Desk Encyclopedia of General Virology|publisher=Academic Press |location=Boston |year=2009 |pages=167–174 |isbn=978-0-12-375146-1}}</ref>{{clear}}

The bacterium ''E. coli'' is the host for [[Escherichia virus T4|bacteriophage T4]] that has a prolate head structure. The bacteriophage encoded gp31 protein appears to be functionally homologous to ''E. coli'' chaperone protein GroES and able to substitute for it in the assembly of bacteriophage T4 virions during infection.<ref name = Marusich1998>Marusich EI, Kurochkina LP, Mesyanzhinov VV. Chaperones in bacteriophage T4 assembly. Biochemistry (Mosc). 1998;63(4):399-406</ref> Like GroES, gp31 forms a stable complex with [[GroEL]] [[chaperone (protein)|chaperonin]] that is absolutely necessary for the folding and assembly ''in vivo'' of the bacteriophage T4 major capsid protein gp23.<ref name = Marusich1998/>