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 vira l nonstructural proteins, the open reading frame (ORF) II and ORF III produ cts (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 contai ning solution, are allowed to acquire a preincubated mixture of P3 and viri ons in a second feed, thus suggesting a direct interaction between P3 and c oat protein. Herein we demonstrate that P3 directly interacts with purified viral particles and unassembled coat protein without the need for any othe r factor and that P3 mediates the association of P2 with purified virus par ticles. The interaction domain of P3 is located in its C-terminal half, dow nstream of the P3-P2 interaction domain but overlapping a region which bind s nucleic acids, Mutagenesis of P3 which interferes with the interaction be tween P3 and virions is correlated with the loss of transmission by aphids, Taken together, our results demonstrate that P3 plays a crucial role in th e formation of the CaMV transmissible complex by serving as a bridge betwee n P2 and virus particles.