Limited proteolysis of potato virus X by trypsin and plant proteases.

The effect of method of purification and incubation with trypsin on the amino acid composition and molecular weight of potato virus X (PVX) protein was determined. PVX wae prepared by four methods, two of which included incubation of unclarified plant sap at room temperature for 24 hr before addition of organic solvents. The molecular weights of the proteins of PVX determined by SDS polyacrylamide electrophoresis prepared by all four methods was approximately 29,000. The amino acid composition of the proteins of PVX prepared by these methods were similar to previously published compositions of PVX. Upon incubation with trypsin 19 amino acid residues were cleaved from the virus protein without disassembly of the virus. The resulting protein had a molecular weight of 27,000 and a distinctive amino acid composition. Evidence for the action of plant proteases on PVX was found when protein prepared from virus purified by a fifth method showed two components in SDS polyacrylamide gel electrophoresis. The major and minor components had molecular weights of 27,000 and 29,000, respectively. The amino acid composition of this preparation was similar to that of PVX protein obtained after trypsin action. Electron microscopy of some trypsin-treated PVX preparations showed an unusual twisting of particles around each other.