Cowpea Chlorotic Mottle Virus

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Revision as of 04:26, 30 November 2011

Self-Assembling Cowpea Chlorotic Mottle Virus Capsid: Nanoreactor and Scaffold for Molecular Synthesis

Introduction

General Capsid Structure

The viral capsid of CCMV is a complex of proteins stabilized by metal coordination between capsomeres and RNA binding on its internal surface. The viral genome encodes three capsid proteins that are chemically identical. These subunits, arbitrarily called A, B, and C, dimerize with each other and assemble into into hexamers and pentamers that comprise the viral capsomeres.^[1] The complete capsid structure takes the form of a truncated icosahedron (20 faces). It is described as a T=3 capsid, where the T value describes the number of structural units per equilateral face of the icosahedron.[1]

Capsomere Structure

The hexameric capsomeres are formed by the B and C subunits. The N-terminus arms (residue 27 through 49) of the subunits intertwine to form a parallel hexamer beta barrel.^[1] The N-terminus is therefore key to the hexamer composition.^[1]

The atoms of residues 27-49 are colored blue for clarity.

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 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. 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.

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