Identity and location of a minor protein component in virions of southern bean mosaic virus

Abstract

Polyacrylamide-gel electrophoretic analysis of southern bean mosaic virus (SBMV) protein isolated with different procedures revealed the presence of a minor protein component (MW 66,000) besides the major or capsid protein (MW 29,000). The amount of the minor protein in virions was ca. one-tenth the amount of the major protein. The minor SBMV protein remained stable when heated in the presence of sodium dodecyl sulfate, urea, and reducing agents (β-mercaptoethanol or dithiothreitol) or upon succinylation, reduction-carboxymethylation, or performic acid oxidation. Controlled virion disassembly revealed that all of the minor protein and some major protein subunits were intimately linked to the SBMV-RNA. The two proteins were separated and purified by preparative gel electrophoresis for subsequent characterization. The minor and major SBMV proteins possessed an identical amino acid composition. In gel diffusion tests, the minor and the major proteins developed twp strong and confluent precipitin bands when reacted with an antiserum prepared against the purified major protein. Treating purified SBMV major protein with cross-linking reagents (dimethyl adipimidate, formaldehyde, or glutaraldehyde) resulted in the formation of a protein component comigrating with the minor protein isolated from the virions. When SBMV virions were iodinated (Na 125I) using lactoperoxidase or chloramine-T, radioactivity was incorporated into the minor and major proteins. These results suggest that the minor SBMV protein is a covalently linked coat protein dimer involving ϵ-amino groups of lysyl residues and resides in the virion capsid. On the basis of these studies, a tentative structural model for the SBMV capsid is proposed.