Mutations in the N-terminal kinase-like domain of the repressor of photomorphogenesis SPA1 severely impair SPA1 function but not light responsiveness in Arabidopsis

Plant J. 2016 Oct;88(2):205-218. doi: 10.1111/tpj.13241. Epub 2016 Aug 29.


The COP1/SPA complex is an E3 ubiquitin ligase that acts as a key repressor of photomorphogenesis in dark-grown plants. While both COP1 and the four SPA proteins contain coiled-coil and WD-repeat domains, SPA proteins differ from COP1 in carrying an N-terminal kinase-like domain that is not present in COP1. Here, we have analyzed the effects of deletions and missense mutations in the N-terminus of SPA1 when expressed in a spa quadruple mutant background devoid of any other SPA proteins. Deletion of the large N-terminus of SPA1 severely impaired SPA1 activity in transgenic plants with respect to seedling etiolation, leaf expansion and flowering time. This ΔN SPA1 protein showed a strongly reduced affinity for COP1 in vitro and in vivo, indicating that the N-terminus contributes to COP1/SPA complex formation. Deletion of only the highly conserved 95 amino acids of the kinase-like domain did not severely affect SPA1 function nor interactions with COP1 or cryptochromes. In contrast, missense mutations in this part of the kinase-like domain severely abrogated SPA1 function, suggesting an overriding negative effect of these mutations on SPA1 activity. We therefore hypothesize that the sequence of the kinase-like domain has been conserved during evolution because it carries structural information important for the activity of SPA1 in darkness. The N-terminus of SPA1 was not essential for light responsiveness of seedlings, suggesting that photoreceptors can inhibit the COP1/SPA complex in the absence of the SPA1 N-terminal domain. Together, these results uncover an important, but complex role of the SPA1 N-terminus in the suppression of photomorphogenesis.

Keywords: Arabidopsis thaliana; COP1; SPA1; flowering time; light signaling; photomorphogenesis; ubiquitin ligase.

MeSH terms

  • Arabidopsis / genetics
  • Arabidopsis / metabolism*
  • Arabidopsis / radiation effects
  • Arabidopsis Proteins / chemistry
  • Arabidopsis Proteins / genetics
  • Arabidopsis Proteins / metabolism*
  • Cell Cycle Proteins / chemistry
  • Cell Cycle Proteins / genetics*
  • Cell Cycle Proteins / metabolism*
  • Flowers / genetics
  • Flowers / metabolism
  • Flowers / radiation effects
  • Gene Expression Regulation, Plant
  • Light*
  • Mutation / genetics
  • Ubiquitin-Protein Ligases


  • Arabidopsis Proteins
  • Cell Cycle Proteins
  • SPA1 protein, Arabidopsis
  • AT2G32950 protein, Arabidopsis
  • Ubiquitin-Protein Ligases