Effect of C-terminal mutations of alfalfa mosaic virus coat protein on dimer formation and assembly in vitro

The coat protein (CP) of alfalfa mosaic virus (AMV) strain 425 assembles to bacilliform or rod-shaped particles in the presence of nucleic acids or to T = 1 empty icosahedral particles in the absence of nucleic acids. To study the determinants of CP assembly, recombinant CPs (rCPs) that contained a (His), region were expressed in Escherichia coli. Wt rCP and a mutant rCP, which lacked the last nine amino acids of the C terminus (amino acids 213-221), assembled to particles that were identical in electron micrographs. However, a mutant rCP, which lacked the last 18 amino acids of the C terminus (amino acids 204-221), did not assemble. Likewise, a mutant with alanine substitutions at W-191, F-197, and P-198, did not assemble. Furthermore rCP with a single alanine substitution at W-191 did not assemble, whereas the rCP, which had an arginine and an alanine substitution at A(196) and F-197, respectively, formed rod-shaped particles. The mutations that prevented assembly prevented dimer formation, which indicates that dimers are the minimal building blocks of particles. Our results indicate that two separate regions in the C terminus of AMV CP are critical for dimer formation and assembly and that changes in key amino acids in one of the regions affect both assembly and particle morphology. (C) 1999 Academic Press.