Mutations in the N-terminal kinase-like domain of the repressor of photomorphogenesis SPA1 severely impair SPA1 function but not light responsiveness in Arabidopsis.
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.
Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.
Xu
X
Holtkotte
Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.
Stefan
S
Dieterle
Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.
Leonie
L
Kokkelink
Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.
Oliver
O
Artz
Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.
Lisa
L
Leson
Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.
Kirsten
K
Fittinghoff
Max Planck Institute of Plant Breeding Research and Cluster of Excellence on Plant Sciences (CEPLAS), Carl-von-Linné Weg 10, 50829, Cologne, Germany.
Ryosuke
R
Hayama
UMR 8256 (B2A) CNRS - UPMC, IBPS, Université Pierre et Marie Curie, Bat C 3éme étage, 9 quai Saint-Bernard, 75252, Paris, Cedex 05, France.
Margaret
M
Ahmad
Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.
Ute
U
Hoecker
D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism* / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018797:Cell Cycle Proteins / Q000737:chemistry / Q000235:genetics* / Q000378:metabolism*; D035264:Flowers / Q000235:genetics / Q000378:metabolism / Q000528:radiation effects; D018506:Gene Expression Regulation, Plant; D008027:Light; D009154:Mutation / Q000235:genetics; D044767:Ubiquitin-Protein Ligases
D029681:Arabidopsis Proteins; D018797:Cell Cycle Proteins; C118571:SPA1 protein, Arabidopsis; C471983:AT2G32950 protein, Arabidopsis; D044767:Ubiquitin-Protein Ligases

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