The full-length product of cauliflower mosaic virus open reading frame III is associated with the viral particle.

The gene III product (P15) of cauliflower mosaic virus (CaMV) is a DNA binding protein in which the DNA binding activity is located on its C-terminal part. In previous work, a C-terminal processed form of P15 (P11) was detected in purified viral particles as a minor component. The full-length P15 was shown to be present and to be matured, possibly by a cysteine proteinase, in CaMV replication complexes isolated from infected turnip leaves. In this paper, we have shown that a virion-enriched fraction obtained from such replication complexes by size exclusion chromatography contained most of the P15 in its uncleaved form and was enriched in the activity responsible for its proteolysis. This enabled us to characterize better the proteinase activity (temperature and pH optimum; effect of specific inhibitors) responsible for P15 cleavage and to confirm that it corresponds to a cysteine proteinase. Based upon these observations, a purification procedure for CaMV particles was devised which impaired the cleavage of P15 into P11 and allowed the isolation of virions containing almost exclusively the noncleaved form. This finding supports our hypothesis that the CaMV gene III product could be involved in the folding of the viral genome during encapsidation.