Cowpea Chlorotic Mottle Virus: Difference between revisions

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Each hexamer consists of six beta strands that run parallel to each other and result in a channel in the center of the hexamer. Residues 29-33 line this pore and reinforce the hexamer subunits by the interactions of their side chains with adjacent residues. For example, the "side chain oxygens of Gln29  residues hydrogen bond with the main chain nitrogens of adjacent Gln29 residues, making a circular ring of interactions" and the "valine residues stack upon one another inside the the beta-tube forming a circle of hydrophobic bonds." The hydrophobic valine side chain atoms are protected from the interior of the virus by the side chain atoms of the glutamine residue, and they are surrounded by the hydrogen bonding of the beta barrels.  
Each hexamer consists of six beta strands that run parallel to each other and result in a channel in the center of the hexamer. Residues 29-33 line this pore and reinforce the hexamer subunits by the interactions of their side chains with adjacent residues. For example, the "side chain oxygens of Gln29  residues hydrogen bond with the main chain nitrogens of adjacent Gln29 residues, making a circular ring of interactions" and the "valine residues stack upon one another inside the the beta-tube forming a circle of hydrophobic bonds." The hydrophobic valine side chain atoms are protected from the interior of the virus by the side chain atoms of the glutamine residue, and they are surrounded by the hydrogen bonding of the beta barrels.  


In the <scene name='Cowpea_Chlorotic_Mottle_Virus/Interior_of_beta_barrel/1'>interior of beta barrel</scene>we can see these residues filling the interior of the channel formed at the center of the hexamers: glutamine 29 (in red), valine 31 (in orange), and valine 33 (in green).
In the <scene name='Cowpea_Chlorotic_Mottle_Virus/Interior_of_beta_barrel/1'>interior of beta barrel</scene> we can see these residues filling the interior of the channel formed at the center of the hexamers: glutamine 29 (in red), valine 31 (in orange), and valine 33 (in green).


Pentamer capsomeres, on the other hand, are formed exclusively from the contribution of A subunit chains.  
Pentamer capsomeres, on the other hand, are formed exclusively from the contribution of A subunit chains.  
<scene name='Cowpea_Chlorotic_Mottle_Virus/Pentamer/1'>Pentamer capsomeres</scene> also contain the same beta barrel structure but the amino-terminus arms cluster to create 5-fold symmetry. The positively charged Lys 42 residue is colored in blue as a marker for the N-terminus arms. The 41 amino acids before this residue do not have detectable electron density for the techniques used to render the structure.  
<scene name='Cowpea_Chlorotic_Mottle_Virus/Pentamer/1'>Pentamer capsomeres</scene> also contain barrel structures but the amino-terminus arms cluster to create 5-fold symmetry. The positively charged Lys 42 residue is colored in blue as a marker for the N-terminus arms. The 41 amino acids before this residue do not have detectable electron density for the techniques used to render the structure.  
 
Interestingly, hexamer formation predominates in the capsid. As previously mentioned, the icosahedral molecule takes on a T=3 value. Kaspar and Klug predicted a classical model for such a structure- one that would have "a sheet of hexamers interspersed with 12 pentamers arranged to form a closed shell with icosahedral symmetry." Prior to the characterization of CCMV's structure, no RNA viruses (in plants and insects) were found to rigorously observe this model. Their prediction necessitated that the pentamer and hexamer subunits would from a single chemical structure (realized in CCMV due to its identical A,B, and C gene products). However, the "molecular switch" that determines whether a pentamer or a hexamer would form, was left undefined.
It appears that after dimer formation, hexamers predominate in solution. The authors propose that the hexamers then form nucleation sites for particle formation. The basis for the additional hexamer stability comes from the relative number of interactions between molecules.
 
 
 


Interestingly, hexamer formation predominates in the capsid. As previously mentioned, the icosahedral molecule takes on a T=3 value. Kaspar and Klug predicted a classical model for such a structure- one that would have "a sheet of hexamers interspersed with 12 pentamers arranged to form a closed shell with icosahedral symmetry." Prior to the characterization of CCMV's structure, no RNA viruses were found to rigorously observe this model. Their prediction necessitated that the pentamer and hexamer subunits were formed from a single chemical structure (realized in CCMV by its identical A,B, and C gene products). The question of "molecular switch that determines which is formed" was not defined.


It serves as the nucleation point for capsid construction.




*All Structural Content Derived from Speir et al.
*All Structural Content Derived from Speir et al.
"STRUCTURES OF THE NATIVE AND SWOLLEN FORMS OF COWPEA CHLOROTIC MOTTLE VIRUS DETERMINED BY X-RAY CRYSTALLOGRAPHY AND CRYO-ELECTRON MICROSCOPY"
"STRUCTURES OF THE NATIVE AND SWOLLEN FORMS OF COWPEA CHLOROTIC MOTTLE VIRUS DETERMINED BY X-RAY CRYSTALLOGRAPHY AND CRYO-ELECTRON MICROSCOPY"
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