![(A) Structures of RPS4, RRS1-R and RRS1-S. (B) Regulation of the RRS1/RPS4 receptor complex relies on complex inter and intramolecular interactions. Key features of the pre-activation state, in which RPS4 is repressed by RRS1, are (1) interactions between the RRS1 domain 4 (D4) and its WRKY domain (D5) that trap RRS1 in a closed conformation, as well as (2) interactions between the TIR domains of RRS1 and RPS4 that prevent activation of signalling by the RPS4 TIR domain [77,80]. In RRS1-R, the long C-terminal domain 6 (D6) extends to the RRS1-R TIR domain even in the pre-activation state, whereas in the RRS1-S/RPS4 pre-activation state, the TIR domain and the C-terminus (D5D6) are not in close proximity. In both pairs, AvrRps4 promotes interactions between the C terminus (D5D6) and the TIR domain of RRS1, which de-represses the hetero-complex. PopP2 acetylates the WRKY domain (D5) of both RRS1-R and RRS1-S, but is only perceived by RRS1-R through intramolecular TIR/D5D6-R interaction [81]. Effector-triggered TIR/D5D6 interactions in both RRS1-R and -S relieve the inhibitory interaction between the TIR domains of RRS1 and RPS4, allowing self-association of RPS4 TIR domains. In addition, interactions between RRS1 D4 and the RPS4 C-terminal domain (CTD) contribute to the activation of the receptor complex [80]. All these processes are believed to culminate in the formation of a resistosome complex that triggers immune signalling probably by proximity-induced activation of the intrinsic NADase activity of RPS4 TIR, a mechanism illustrated by the crystal structures of the TNLs ROQ1 and RPP1 [78–81]. Recent work showed that the phosphorylation of specific threonine and serine residues in the C-terminus of RRS1-R plays an important role in its regulation and PopP2 responsiveness [81]. The phosphorylation of one specific threonine residue of the WRKY domain is required to maintain RRS1-R in its inactive state. Phosphorylation at other sites located in D6 strengthens the interaction of the C terminus and the TIR domain of RRS1 and is required for activation by PopP2 but not for AvrRps4 responsiveness. P: phosphorylation, A: acetylation, E: effector-triggered modification or binding.](https://port.silverchair-cdn.com/port/content_public/journal/essaysbiochem/66/5/10.1042_ebc20210079/2/ebc-2021-0079ci004.png?Expires=1716285220&Signature=GFmRGDt7R2PPbnBV-pY5kY8Q-tcS3BQe~67zbEMPosIpZ7j05PwLKwUVGWfe7N7A~cGeYXmQldKYZ3-iUuIIyChVLfPWDLvR80HI0GvlhRk4cSFJnjklTwhRa9HUrnDUuVDkd0IEzcpM01MkJYloBNUsAmAVrte5jma7lt9lpj1IvNgu1psBoEZ7cNknWe3TraK13W-mAkr7nmtt3~DCNcRq6UgsAPN1yXyCw7EF~c7RkO6XKHmRSC~1uuOrrsSGD7FbMb7pN~ChtD6V9j2KeUN~53awQUhGNlHqSqooXxPt6rTlrC-OEW7EBYbkjE2Udt~C3S067iIFL3dSFbuEnQ__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA)
(A) Structures of RPS4, RRS1-R and RRS1-S. (B) Regulation of the RRS1/RPS4 receptor complex relies on complex inter and intramolecular interactions. Key features of the pre-activation state, in which RPS4 is repressed by RRS1, are (1) interactions between the RRS1 domain 4 (D4) and its WRKY domain (D5) that trap RRS1 in a closed conformation, as well as (2) interactions between the TIR domains of RRS1 and RPS4 that prevent activation of signalling by the RPS4 TIR domain [77,80]. In RRS1-R, the long C-terminal domain 6 (D6) extends to the RRS1-R TIR domain even in the pre-activation state, whereas in the RRS1-S/RPS4 pre-activation state, the TIR domain and the C-terminus (D5D6) are not in close proximity. In both pairs, AvrRps4 promotes interactions between the C terminus (D5D6) and the TIR domain of RRS1, which de-represses the hetero-complex. PopP2 acetylates the WRKY domain (D5) of both RRS1-R and RRS1-S, but is only perceived by RRS1-R through intramolecular TIR/D5D6-R interaction [81]. Effector-triggered TIR/D5D6 interactions in both RRS1-R and -S relieve the inhibitory interaction between the TIR domains of RRS1 and RPS4, allowing self-association of RPS4 TIR domains. In addition, interactions between RRS1 D4 and the RPS4 C-terminal domain (CTD) contribute to the activation of the receptor complex [80]. All these processes are believed to culminate in the formation of a resistosome complex that triggers immune signalling probably by proximity-induced activation of the intrinsic NADase activity of RPS4 TIR, a mechanism illustrated by the crystal structures of the TNLs ROQ1 and RPP1 [78–81]. Recent work showed that the phosphorylation of specific threonine and serine residues in the C-terminus of RRS1-R plays an important role in its regulation and PopP2 responsiveness [81]. The phosphorylation of one specific threonine residue of the WRKY domain is required to maintain RRS1-R in its inactive state. Phosphorylation at other sites located in D6 strengthens the interaction of the C terminus and the TIR domain of RRS1 and is required for activation by PopP2 but not for AvrRps4 responsiveness. P: phosphorylation, A: acetylation, E: effector-triggered modification or binding.