Proteins of cowpea chlorotic mottle, broad bean mottle, and brome mosaic viruses

Abstract

Molecular weights of virus protein subunits of cowpea chlorotic mottle virus (CCMV), broad bean mottle virus (BBMV), and brome mosaic virus (BMV) were determined by means of polyacrylamide gel electrophoresis. Values of 19,200 for CCMV, 16,400 for BBMV, and 16,200 for BMV were obtained. Amino acid compositions of the three viruses have been determined and compared with values reported earlier. Based on these data CCMV, BBMV, and BMV were found to possess 179, 152, and 150 amino acid residues per protein subunit, respectively. The CCMV subunit on trypsin treatment gave one component with a molecular weight of 16,400 and 154 residues. Unlike CCMV in which only certain lysyl and arginyl bonds in the N-terminal portion of the protein are hydrolyzed by trypsin, BBMV did not show any selective response and some of the virus particles were completely digested under the same conditions. Thus the BBMV protein subunits are much more susceptible to tryptic action than those of CCMV. Chymotrypsin treatment of BBMV hydrolyzed some virus particles completely while the protein of a small fraction was only partially hydrolyzed, suggesting the presence of peptide bonds susceptible to attack by enzyme in either terminal of the molecule. Trypsin treatment of BMV produced at least four components. A stable spherical particle of 16 nm diameter (36 S) with an amino acid composition different from that of the virus and containing 6% RNA was isolated after gel chromatography and density gradient centrifugation. However, a precipitate that formed during the digestion was found to contain three components with molecular weights of 11,000, 9700, and 8800. Chymotrypsin treatment under similar conditions had no effect on this virus.