Structural Basis for Assembly and Function of a Heterodimeric Plant Immune Receptor

Abstract

Cytoplasmic plant immune receptors recognize specific pathogen effector proteins and initiate effector-triggered immunity. In Arabidopsis, the immune receptors RPS4 and RRS1 are both required to activate defense to three different pathogens. We show that RPS4 and RRS1 physically associate. Crystal structures of the N-terminal Toll-interleukin-1 receptor/resistance (TIR) domains of RPS4 and RRS1, individually and as a heterodimeric complex (respectively at 2.05, 1.75, and 2.65 angstrom resolution), reveal a conserved TIR/TIR interaction interface. We show that TIR domain heterodimerization is required to form a functional RRS1/RPS4 effector recognition complex. The RPS4 TIR domain activates effector-independent defense, which is inhibited by the RRS1 TIR domain through the heterodimerization interface. Thus, RPS4 and RRS1 function as a receptor complex in which the two components play distinct roles in recognition and signaling.

Two linked pairs of Arabidopsis TNL resistance genes independently confer recognition of bacterial effector AvrRps4

“…We showed overexpression of RPS4 TIR (1-235 a.a.)-triggered cell death in N. tabacum, whereas RRS1 TIR (1-175 a.a.) did not (Fig. 6a), consistent with previous report 24 . Interestingly, neither RPS4B TIR (1-235 a.a.) nor RRS1B TIR (1-166 a.a.) triggered cell death when overexpressed in N. tabacum (Fig.…”

“…Co-expression of avrRps4 or popP2, together with RRS1 and RPS4, by agro-infiltration in N. tabacum leaves results in HR 24 . Using a similar method, we were able to show AvrRps4 recognition by RRS1B/RPS4B in N. tabacum.…”

“…Many lines of evidence suggest TIR-TIR interactions are important for TNL function 24,37 . The TIR domains of RRS1 and RPS4 interact in a yeast two hybrid assay and associate in planta after co-IP 24 .…”

“…RRS1 and RPS4 physically associate to form a functional recognition complex in planta 24 . Therefore, we tested by coimmunoprecipitation (co-IP) whether RRS1B and RPS4B also associate in planta, and if this association is specific to pair partners.…”

“…The RRS1B and RPS4B proteins associate with each other, as previously reported for RRS1 and RPS4 (ref. 24). Non-authentic R protein associations can also occur between RRS1B and RPS4, and RRS1 and RPS4B, but do not enable effector recognition, suggesting that these R proteins must pair with their appropriate respective partner for function.…”