Interaction between the open reading frame III product and the coat protein is required for transmission of cauliflower mosaic virus by aphids

Transmission of cauliflower mosaic virus (CaMV) by aphids requires two viral nonstructural proteins, the open reading frame (ORF) II and ORF III products (P2 and P3). An interaction between a C-terminal domain of P2 and an N-terminal domain of P3 is essential for transmission. Purified particles of CaMV are efficiently transmitted only if aphids, previously fed a P2-containing solution, are allowed to acquire a preincubated mixture of P3 and virions in a second feed, thus suggesting a direct interaction between P3 and coat protein. Herein we demonstrate that P3 directly interacts with purified viral particles and unassembled coat protein without the need for any other factor and that P3 mediates the association of P2 with purified virus particles. The interaction domain of P3 is located in its C-terminal half, downstream of the P3-P2 interaction domain but overlapping a region which binds nucleic acids. Mutagenesis of P3 which interferes with the interaction between P3 and virions is correlated with the loss of transmission by aphids. Taken together, our results demonstrate that P3 plays a crucial role in the formation of the CaMV transmissible complex by serving as a bridge between P2 and virus particles.