Assembly and crystallization of a T = 1 icosahedral particle from trypsinized southern bean mosaic virus coat protein.

The N-terminal 61 amino acid residues of the coat protein of southern bean mosaic virus were removed by mild trypsinolysis, resulting in a 22,000 MW fragment, P22. In the absence of nucleic acid, the cleaved protein could be assembled into 175 A diameter, T = 1, spherical particles. These particles were formed between pH 4 and 7, at low ionic strength (I </= 0.01), and also between pH 8 and 11 when Ca2+ ions were present. Mg2+ ions had no effect on the assembly of these spheres. The T = 1 particles, assembled with Ca2+ ion, were crystallized by vapor diffusion in ammonium sulfate at pH 9.4. The cubic crystals diffracted to at least 4 A resolution. The three particles in the P23 unit cell contained 222 crystallographic symmetry.