title: The functional divergence between SPA1 and SPA2 in Arabidopsis photomorphogenesis maps primarily to the respective N-terminal kinase-like domain.
abstract: Plants have evolved complex mechanisms to adapt growth and development to the light environment. The COP1/SPA complex is a key repressor of photomorphogenesis in dark-grown Arabidopsis plants and acts as an E3 ubiquitin ligase to ubiquitinate transcription factors involved in the light response. In the light, COP1/SPA activity is inhibited by photoreceptors, thereby allowing accumulation of these transcription factors and a subsequent light response. Previous results have shown that the four members of the SPA family exhibit partially divergent functions. In particular, SPA1 and SPA2 strongly differ in their responsiveness to light, while they have indistinguishable activities in darkness. The much higher light-responsiveness of SPA2 is partially explained by the much stronger light-induced degradation of SPA2 when compared to SPA1. Here, we have conducted SPA1/SPA2 domain swap experiments to identify the protein domain(s) responsible for the functional divergence between SPA1 and SPA2.
  
  
  
  We have individually swapped the three domains between SPA1 and SPA2 - the N-terminal kinase-like domain, the coiled-coil domain and the WD-repeat domain - and expressed them in spa mutant Arabidopsis plants. The phenotypes of transgenic seedlings show that the respective N-terminal kinase-like domain is primarily responsible for the respective light-responsiveness of SPA1 and SPA2. Furthermore, the most divergent part of the N-terminal domain was sufficient to confer a SPA1- or SPA2-like activity to the respective SPA protein. The stronger light-induced degradation of SPA2 when compared to SPA1 was also primarily conferred by the SPA2 N-terminal domain. At last, the different affinities of SPA1 and SPA2 for cryptochrome 2 are defined by the N-terminal domain of the respective SPA protein. In contrast, both SPA1 and SPA2 similarly interacted with COP1 in light-grown seedlings.
  
  
  
  Our results show that the distinct activities and protein stabilities of SPA1 and SPA2 in light-grown seedlings are primarily encoded by their N-terminal kinase-like domains. Similarly, the different affinities of SPA1 and SPA2 for cry2 are explained by their respective N-terminal domain. Hence, after a duplication event during evolution, the N-terminal domains of SPA1 and SPA2 underwent subfunctionalization, possibly to allow optimal adaptation of growth and development to a changing light environment.
authors.affiliation: Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.|Present Address: Department of Botany and Plant Biology, University of Geneva, Sciences III, 30 Quai E. Ansermet, 1211, Geneva 4, Switzerland.
authors.forename: Song
authors.initials: S
authors.lastname: Chen
authors.affiliation: Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.|Present Address: Department of Plant Biochemistry, Dahlem Center of Plant Sciences, Freie Universität Berlin, Königin-Luise-Str. 12-16, Berlin, Germany.
authors.forename: Lennart
authors.initials: L
authors.lastname: Wirthmueller
authors.affiliation: Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.
authors.forename: Johannes
authors.initials: J
authors.lastname: Stauber
authors.affiliation: Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.
authors.forename: Niels
authors.initials: N
authors.lastname: Lory
authors.affiliation: Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.
authors.forename: Xu
authors.initials: X
authors.lastname: Holtkotte
authors.affiliation: Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.
authors.forename: Lisa
authors.initials: L
authors.lastname: Leson
authors.affiliation: Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany.
authors.forename: Christian
authors.initials: C
authors.lastname: Schenkel
authors.affiliation: UMR 8256 (B2A) CNRS - UPMC, IBPS, Université Pierre et Marie Curie, Bat C, 9 quai Saint-Bernard, 75252, Paris Cedex 05, France.
authors.forename: Margaret
authors.initials: M
authors.lastname: Ahmad
authors.affiliation: Botanical Institute and Cluster of Excellence on Plant Sciences (CEPLAS), Biocenter, University of Cologne, Zülpicher Str. 47b, 50674, Cologne, Germany. hoeckeru@uni-koeln.de.
authors.forename: Ute
authors.identifier: 0000-0002-5636-9777
authors.initials: U
authors.lastname: Hoecker
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism* / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D005075:Biological Evolution; D018797:Cell Cycle Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D018506:Gene Expression Regulation, Plant / Q000528:radiation effects; D008027:Light; D000072417:Protein Domains; D017346:Protein Serine-Threonine Kinases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D036226:Seedlings / Q000235:genetics / Q000254:growth & development / Q000378:metabolism / Q000528:radiation effects
chemical_list: D029681:Arabidopsis Proteins; D018797:Cell Cycle Proteins; C118571:SPA1 protein, Arabidopsis; D017346:Protein Serine-Threonine Kinases; C517423:SPA2 protein, Arabidopsis
keywords: Arabidopsis; COP1; E3 ubiquitin ligase; Functional divergence; Photomorphogenesis; SPA1; SPA2