Partial purification and characterization of potato virus Y helper component

Mg2+ stabilized potato virus Y helper component during partial purification by precipitation using polyethyleneglycol. In solutions containing 0.02 M Mg2+ the helper component retained most of its activity for 2 days at 4° and for at least 8 months at −15°. Activity was destroyed on incubation with pronase or trypsin, or by heating for 5 min at 55°, but not by incubation with ribonuclease. Incubation with its own antiserum strongly inhibited helper component activity, but antisera to potato virus Y coat protein or inclusion protein had no more effect than a control serum showing that the component was unrelated to these two proteins. Filtration through a Sephadex G-200 column resulted in a broad peak of activity which produced many protein-staining bands when electrophoresed on polyacrylamide gel. Gel filtration and ultrafiltration experiments both indicated a molecular weight of 100,000–200,000. Some helper component activity was retained by aphids allowed to probe into a sucrose solution for 20 min showing that the helper component is more firmly bound to the aphid than is the tobacco mosaic viruspoly-l-ornithine complex.