Exploring the Role of RanGAP2 in Recognition by the Potato CC-NB-LRR Immune Receptors Rx1 and Gpa2

The Ran GTPase activating protein 2 (RanGAP2) is a co-factor of the potato CC-NB-LRR immune receptor Rx1. However, its role in defence remains to be fully elucidated. Artificial tethering of RanGAP2 to the nematode effector GpRbp-1 was shown to enhance the cell death response of Gpa2, a closely related immune receptor of Rx1 that also interacts with RanGAP2 (Sacco et al. 2009). This suggests that RanGAP2 may contribute to immunity by facilitating effector recognition. Here, we expand on this model using a combination of structure-informed approaches, including co-immunoprecipitation and cellular imaging studies. We show that RanGAP2 can in fact form complexes in planta with the cognate effectors of Rx1 and Gpa2, namely the PVX coat protein (CP) and GpRbp-1. Interestingly, this was noted for both the eliciting and non-eliciting effector variants, suggesting that the RanGAP2/effector interaction may not be sufficient to confer recognition. This is in line with existing data demonstrating that the C-terminal region of the Rx1 and Gpa2 LRR domains is required for successful pathogen perception (Slootweg et al. 2013). Currently, we are performing structural and biochemical analysis to discern whether RanGAP2 acts as a bait that facilitates specific effector recognition either by bridging or enhancing the effector/cognate LRR interaction. This model may explain the bifurcation of pathogen recognition specificities of these two closely related immune receptors during evolution.