title: Mechanism of capsid assembly for an icosahedral plant virus.
abstract: Capsids of spherical viruses share a common architecture: an icosahedral arrangement of identical proteins. We suggest that there may be a limited number of common assembly mechanisms for such viruses. Previous assembly mechanisms were proposed on the basis of virion structure but were not rigorously tested. Here we apply a rigorous analysis of assembly to cowpea chlorotic mottle virus (CCMV), a typical, small, positive-strand RNA virus. The atomic resolution structure of CCMV revealed an interleaving of subunits around the quasi-sixfold vertices, which suggested that capsid assembly was initiated by a hexamer of dimers (Speir et al., 1995, Structure 3, 63-78). However, we find that the capsid protein readily forms pentamers of dimers in solution, based on polymerization kinetics observed by light scattering. Capsid assembly is nucleated by a pentamer, determined from analysis of the extent of assembly by size-exclusion chromatography. Subsequent assembly likely proceeds by the cooperative addition of dimers, leading to the T = 3 icosahedral capsid. At high protein concentrations, the concentration-dependent nucleation reaction causes an overabundance of five-dimer nuclei that can be identified by classical light scattering. In turn these associate to form incomplete capsids and pseudo-T = 2 capsids, assembled by oligomerization of 12 pentamers of dimers. The experimentally derived assembly mechanisms of T = 3 and pseudo-T = 2 CCMV capsids are directly relevant to interpreting the structure and assembly of other T = 3 viruses such as Norwalk virus and pseudo-T = 2 viruses such as the vp3 core of blue tongue virus.
authors.affiliation: Department of Biochemistry and Molecular Biology, University of Oklahoma Health Sciences Center, Oklahoma City, Oklahoma 73190, USA. adam-zlotnick@ouhsc.edu
authors.forename: A
authors.initials: A
authors.lastname: Zlotnick
authors.forename: R
authors.initials: R
authors.lastname: Aldrich
authors.forename: J M
authors.initials: JM
authors.lastname: Johnson
authors.forename: P
authors.initials: P
authors.lastname: Ceres
authors.forename: M J
authors.initials: MJ
authors.lastname: Young
mesh_terms: D017795:Bromovirus / Q000254:growth & development* / Q000378:metabolism / Q000648:ultrastructure; D002213:Capsid / Q000032:analysis / Q000096:biosynthesis*; D002850:Chromatography, Gel; D019281:Dimerization; D008854:Microscopy, Electron; D053830:Plant Viral Movement Proteins; D010944:Plants / Q000821:virology*; D014764:Viral Proteins / Q000096:biosynthesis*
chemical_list: D053830:Plant Viral Movement Proteins; D014764:Viral Proteins