T=1 Capsid Structures of Sesbania Mosaic Virus Coat Protein Mutants: Determinants of T=3 and T=1 Capsid Assembly

Sesbania mosaic virus particles consist of 180 coat protein subunits of 29 kDa organized on a T=3 icosahedral lattice. N-terminal deletion mutants of coat protein that lack 36 (CP-N \Delta 36) and 65 (CP-N \Delta 65) residues from the N terminus, when expressed in Escherichia coli, produced similar T=1 capsids of approximate diameter 20 nm. In contrast to the wild-type particles, these contain only 60 copies of the truncated protein subunits (T=1). CP-N \Delta 65 lacks the \beta-annulus believed to be responsible for the error-free assembly of T=3 particles. Though the CP-N \Delta 36 mutant has the b-annulus segment, it does not form a T=3 capsid, presumably because it lacks an arginine-rich motif found close to the amino terminus. Both CPND36 and CP-N \Delta 65 T=1 capsids retain many key features of the T=3 quaternary structure. Calcium binding geometries at the coat protein interfaces in these two particles are also nearly identical. When the conserved aspartate residues that coordinate the calcium, D146 and D149 in the CP-ND65, were mutated to asparagine (CP-N \Delta 65-D146N-D149N), the subunits assembled into T=1 particles but failed to bind calcium ions. The structure of this mutant revealed particles that were slightly expanded. The analysis of the structures of these mutant capsids suggests that although calcium binding contributes substantially to the stability of T=1 particles, it is not mandatory for their assembly. In contrast, the presence of a large fraction of the amino-terminal arm including sequences that precede the \beta-annulus and the conserved D149 appear to be indispensable for the error-free assembly of T=3 particles.