Functional comparison of the WD-repeat domains of SPA1 and COP1 in suppression of photomorphogenesis

Plant Cell Environ. 2021 Oct;44(10):3273-3282. doi: 10.1111/pce.14148. Epub 2021 Jul 21.


The Arabidopsis COP1/SPA complex acts as a cullin4-based E3 ubiquitin ligase to suppress photomorphogenesis in darkness. It is a tetrameric complex of two COP1 and two SPA proteins. Both COP1 and SPA are essential for the activity of this complex, and they both contain a C-terminal WD-repeat domain responsible for substrate recruitment and binding of DDB1. Here, we used a WD domain swap-approach to address the cooperativity of COP1 and SPA proteins. We found that expression of a chimeric COP1 carrying the WD-repeat domain of SPA1 mostly complemented the cop1-4-mutant phenotype in darkness, indicating that the WD repeat of SPA1 can replace the WD repeat of COP1. In the light, SPA1-WD partially substituted for COP1-WD. In contrast, expression of a chimeric SPA1 protein carrying the WD repeat of COP1 did not rescue the spa-mutant phenotype. Together, our findings demonstrate that a SPA1-type WD repeat is essential for COP1/SPA activity, while a COP1-type WD is in part dispensible. Moreover, a complex with four SPA1-WDs is more active than a complex with only two SPA1-WDs. A homology model of SPA1-WD based on the crystal structure of COP1-WD uncovered two insertions and several amino acid substitutions at the predicted substrate-binding pocket of SPA1-WD.

Keywords: Arabidopsis; light signal transduction; ubiquitin ligase.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Arabidopsis / genetics*
  • Arabidopsis / growth & development*
  • Arabidopsis / metabolism
  • Arabidopsis / radiation effects
  • Arabidopsis Proteins / genetics*
  • Arabidopsis Proteins / metabolism
  • Cell Cycle Proteins / genetics*
  • Cell Cycle Proteins / metabolism
  • Coat Protein Complex I / genetics*
  • Coat Protein Complex I / metabolism
  • Plant Development / radiation effects
  • WD40 Repeats / genetics*


  • Arabidopsis Proteins
  • Cell Cycle Proteins
  • Coat Protein Complex I
  • SPA1 protein, Arabidopsis