Regulated Disorder: Posttranslational Modifications Control the RIN4 Plant Immune Signaling Hub

Abstract

RIN4 is an intensively studied immune regulator in Arabidopsis and is involved in perception of microbial features outside and bacterial effectors inside plant cells. Furthermore, RIN4 is conserved in land plants and is targeted for posttranslational modifications by several virulence proteins from the bacterial pathogen Pseudomonas syringae. Despite the important roles of RIN4 in plant immune responses, its molecular function is not known. RIN4 is an intrinsically disordered protein (IDP), except at regions where pathogen-induced posttranslational modifications take place. IDP act as hubs for protein complex formation due to their ability to bind to multiple client proteins and, thus, are important players in signal transduction pathways. RIN4 is known to associate with multiple proteins involved in immunity, likely acting as an immune-signaling hub for the formation of distinct protein complexes. Genetically, RIN4 is a negative regulator of immunity, but diverse posttranslational modifications can either enhance its negative regulatory function or, on the contrary, render it a potent immune activator. In this review, we describe the structural domains of RIN4 proteins, their intrinsically disordered regions, posttranslational modifications, and highlight the implications that these features have on RIN4 function. In addition, we will discuss the potential role of plasma membrane subdomains in mediating RIN4 protein complex formations.

Fig. 1.

RIN4 proteins, their domain architecture, sites of posttranslational modification, and regions of intrinsic disorder. Schematic diagram of Arabidopsis RIN4 (AtRIN4), soybean RIN4 (GmRIN4a), lettuce RIN4 (LsRIN4), and tomato RIN4 (SlRIN4) proteins. Positions of N-terminal NOI, C-terminal NOI, and membrane-anchoring domains are indicated as light green, dark green and orange blocks, respectively. Indicated are phosphorylated and acetylated residues and AvrRpt2-cleavage sites of AtRIN4 and their conservation in other RIN4 homologs. Intrinsically disordered regions indicated for each RIN4 homolog were predicted, using the PrDOS (protein disorder prediction system) server.

Fig. 2.

RIN4-associated proteins are enriched in detergent-resistant membranes (DRM), which are heterologous regions rich in sphingolipids and sterols and may constitute plasma membrane microdomains that serve as platforms for protein complex formation. A, Type III effectors from Pseudomonas syringae trigger nucleotide-binding leucine-rich repeat receptor (NLR)-triggered immunity (NTI) mediated by the RPS2 or RPM1 NLR through AvrRpt2-induced proteolysis of RIN4 or AvrB/AvrRpm1-induced phosphorylation of RIN4 T166, respectively. RIN4 is phosphorylated by RIPK and additional receptor-like cytoplasmic kinases (RLCK). The HopZ3 effector is an acetyltransferase that acetylates the RIN4 complex to inhibit NTI triggered by RPM1. B, Effectors targeting RIN4 suppress PTI promoted by phosphorylation of S141 through AvrB/AvrRpm1-induced phosphorylation of T166. C, The contribution of RIN4 to activity of plasma membrane H⁺-ATPases, which promote stomatal opening, can be enhanced by AvrB/AvrRpm1-induced phosphorylation of T166 or inhibited by ubiquitin-dependent degradation of RIN4 induced by general control non-repressible-4 (GCN4).