SOBIR1 requires the GxxxG dimerization motif in its transmembrane domain to form constitutive complexes with receptor‐like proteins

Abstract

Receptor-like proteins (RLPs), forming an important group of transmembrane receptors in plants, play roles in development and immunity. RLPs contain extracellular leucine-rich repeats (LRRs) and, in contrast with receptor-like kinases (RLKs), lack a cytoplasmic kinase required for the initiation of downstream signalling. Recent studies have revealed that the RLK SOBIR1/EVR (SUPPRESSOR OF BIR1-1/EVERSHED) specifically interacts with RLPs. SOBIR1 stabilizes RLPs and is required for their function. However, the mechanism by which SOBIR1 associates with RLPs and regulates RLP function remains unknown. The Cf immune receptors of tomato (Solanum lycopersicum), mediating resistance to the fungus Cladosporium fulvum, are RLPs that also interact with SOBIR1. Here, we show that both the LRR and kinase domain of SOBIR1 are dispensable for association with the RLP Cf-4, whereas the highly conserved GxxxGxxxG motif present in the transmembrane domain of SOBIR1 is essential for its interaction with Cf-4 and additional RLPs. Complementation assays in Nicotiana benthamiana, in which endogenous SOBIR1 levels were knocked down by virus-induced gene silencing, showed that the LRR domain as well as the kinase activity of SOBIR1 are required for the Cf-4/Avr4-triggered hypersensitive response (HR). In contrast, the LRRs and kinase activity of SOBIR1 are not required for facilitation of Cf-4 accumulation. Together, these results suggest that, in addition to being a stabilizing scaffold for RLPs, SOBIR1 is also required for the initiation of downstream signalling through its kinase domain.

Kinase activity of SOBIR1 and BAK1 is required for immune signalling

“…In agreement with this, SOBIR1 kinase activity is essential for its function downstream of Cf‐4, as it has been shown that a kinase‐dead mutant of SOBIR1 is unable to complement Cf‐4/Avr4 signalling and endocytosis of the Cf‐4/SOBIR1 complex (Bi et al. , ; Liebrand et al. , ; Postma et al.…”

The tomato I gene for Fusarium wilt resistance encodes an atypical leucine‐rich repeat receptor‐like protein whose function is nevertheless dependent on SOBIR1 and SERK3/BAK1

“…A second transmembrane domain would leave a loop of only five residues exposed to the cytosol and only three residues exposed to the apoplast at the C‐terminus. Neither transmembrane domain carries the GxxxG motif typical of the transmembrane domains of most other LRR‐RLPs involved in plant defence (Figure S9; Bi et al ., ).…”

“…Bi et al . () also showed that the LRR domain of SOBIR1 is required for Cf‐4 function. This could suggest that the LRR domain of SOBIR1 interacts with LRR‐RLPs in a similar manner to SERK3/BAK1 and SERK1, but via a different face of the extended superhelix formed by the C‐terminal LRRs and LRR‐flanking domain of the LRR‐RLPs (Figure b); a possibility that also requires further investigation.…”

“…Bi et al . () hypothesised that SOBIR1 interacts with LRR‐RLPs via single or tandem GxxxG motifs present in the transmembrane domains of most LRR‐RLPs, but I lacks any such GxxxG motif and Zhang et al . () have shown that the GxxxG motif present in Ve1 is not required for function.…”

Identification of I‐7 expands the repertoire of genes for resistance to Fusarium wilt in tomato to three resistance gene classes

“…Putative N ‐glycosylation sites are underlined and a putative endocytosis motif is highlighted in green. A possible SOBIR1‐dimerization motif (G xxx G xxx G; Bi et al ., ) is highlighted in light blue. Amino acids present in I ‐7, but missing from i‐7 M82 , owing to deletions in i‐7 relative to I ‐7 , are double underlined, whereas those missing from i‐7 LA716 are overlined.…”