Limited proteolysis of alfalfa mosaic virus: influence on the structural and biological function of the coat protein

Limited tryptic digestion of intact alfalfa mosaic virus resulted in the quantitative removal of 27 amino acids from the N-terminal portions of the coat protein subunits. The release of this peptide material, which contains a relatively high number of basic residues, causes a breakdown of the bacilliform viral components into spherical nucleoprotein particles. From this it was concluded that the proteolytic cleavage interferes with protein-RNA interactions in the virus, but not with protein-protein interactions. The release of the N-terminal peptide also destroyed the capacity of the coat protein to activate the alfalfa mosaic virus genome. This supports the hypothesis that this activation is accomplished by a specific interaction of the coat protein with alfalfa mosaic virus RNAs.