A Regulatory Module Controlling Homeostasis of a Plant Immune Kinase

Mol Cell. 2018 Feb 1;69(3):493-504.e6. doi: 10.1016/j.molcel.2017.12.026. Epub 2018 Jan 18.


Plant pattern recognition receptors (PRRs) perceive microbial and endogenous molecular patterns to activate immune signaling. The cytoplasmic kinase BIK1 acts downstream of multiple PRRs as a rate-limiting component, whose phosphorylation and accumulation are central to immune signal propagation. Previous work identified the calcium-dependent protein kinase CPK28 and heterotrimeric G proteins as negative and positive regulators of BIK1 accumulation, respectively. However, mechanisms underlying this regulation remain unknown. Here we show that the plant U-box proteins PUB25 and PUB26 are homologous E3 ligases that mark BIK1 for degradation to negatively regulate immunity. We demonstrate that the heterotrimeric G proteins inhibit PUB25/26 activity to stabilize BIK1, whereas CPK28 specifically phosphorylates conserved residues in PUB25/26 to enhance their activity and promote BIK1 degradation. Interestingly, PUB25/26 specifically target non-activated BIK1, suggesting that activated BIK1 is maintained for immune signaling. Our findings reveal a multi-protein regulatory module that enables robust yet tightly regulated immune responses.

Keywords: Arabidopsis; Botrytis cinerea; Pseudomonas syringae; calcium-dependent protein kinases; heterotrimeric G proteins; innate immunity; phosphorylation; ubiquitination.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Arabidopsis / metabolism
  • Arabidopsis Proteins / metabolism*
  • Cytoplasm
  • Cytosol
  • Gene Expression Regulation, Plant / genetics
  • Homeostasis
  • Phosphorylation
  • Plant Immunity / physiology
  • Plant Proteins
  • Protein Kinases / metabolism*
  • Protein Serine-Threonine Kinases / metabolism*
  • Signal Transduction
  • Transcription Factors
  • Ubiquitin-Protein Ligases / metabolism*


  • Arabidopsis Proteins
  • Plant Proteins
  • Transcription Factors
  • Ubiquitin-Protein Ligases
  • Protein Kinases
  • calcium-dependent protein kinase
  • BIK1 protein, Arabidopsis
  • Protein Serine-Threonine Kinases