The N-terminus of the Cauliflower mosaic virus aphid transmission protein P2 is involved in both transmission and microtubule interaction
Résumé
To move between hosts, many plant viruses including Cauliflower mosaic virus (CaMV) use aphids as vectors. When the insects feed on infected leaves, virus particles already present in the leaf attach to their mouth parts (stylets). This enables the viruses to be transferred to a new host plant when the aphids change plants. As a prerequisite for CaMV transmission, the viral helper proteinP2 acts as a molecular linker and mediates binding of the virus particles to the aphid stylets. P2 is available in infected plant cells in a specific structure that is formed beforehand during CaMV infection. This structure is specialized for transmission and named the transmission body (TB).When inserting its stylets into an infected leaf, the aphid triggers ultra-rapid massive influx of tubulin into the TB, followed by its disruption. As a consequence, P2 is redistributed on to cortical micro tubules, together with viral particles (that are simultaneously set free from intracellular storage sites) and forms the so-called mixed networks (MNs). The MNs are the predominant structure from which virus is acquired by aphids; inhibiting their formation reduces drastically transmission rates. To find out which are the P2 motifs and domains involved in MN formation, we generated a set of P2 mutants by Alanine scanning. These P2 mutants were analyzed in vitro and in planta (in and without viral context) for their capacity to interact with microtubules and to transmit the virus. One knock-out mutant with N-terminal alanine substitutions lost the capacity to form MNs and to support transmission. Other mutants had altered kinetics of MN formation and transmission capacity, P2 protein stability and phenotypes of TB and viral factories. The results allow to correlate P2's capacity for network formation and microtubule-binding with transmission, confirming the importance of MNs for transmission.