Aphid transmission of cauliflower mosaic virus requires the viral PIII protein

The open reading frame (ORF) III product (PIII) of cauliflower mosaic virus is necessary for the infection cycle but its role is poorly understood. We have used in vitro protein binding ('far Western') assays to demonstrate t hat PIII interacts with the cauliflower mosaic virus (CaMV) ORF II product (PII), a known aphid transmission factor. Aphid transmission of purified vi rions of the PII-defective strain CM4-184 was dependent upon added PII, but complementation was efficient only in the presence of PIII, demonstrating the requirement of PIII for transmission. Deletion mutagenesis mapped the i nteraction domains of PIII and PII to the 30 N-terminal and 61 C-terminal r esidues of PIII and PII, respectively. A model for interaction between PIII and PII is proposed on the basis of secondary structure predictions. Final ly, a direct correlation between the ability of PIII and PII to interact an d aphid transmissibility of the virus was demonstrated by using mutagenized PIII proteins. Taken together, these data argue strongly that PIII is a se cond 'helper' factor required for CaMV transmission by aphids.