title: Light exposure of Arabidopsis seedlings causes rapid de-stabilization as well as selective post-translational inactivation of the repressor of photomorphogenesis SPA2.
abstract: The COP1/SPA complex acts as an E3 ubiquitin ligase to repress photomorphogenesis by targeting activators of the light response for degradation. Genetic analysis has shown that the four members of the SPA gene family (SPA1-SPA4) have overlapping but distinct functions. In particular, SPA1 and SPA2 differ in that SPA1 encodes a potent repressor in light- and dark-grown seedlings, but SPA2 fully loses its function when seedlings are exposed to light, indicating that SPA2 function is hyper-inactivated by light. Here, we have used chimeric SPA1/SPA2 constructs to show that the distinct functions of SPA1 and SPA2 genes in light-grown seedlings are due to the SPA protein sequences and independent of the SPA promoter sequences. Biochemical analysis of SPA1 and SPA2 protein levels shows that light exposure leads to rapid proteasomal degradation of SPA2, and, more weakly, of SPA1, but not of COP1. This suggests that light inactivates the COP1/SPA complex partly by reducing SPA protein levels. Although SPA2 was more strongly degraded than SPA1, this was not the sole reason for the lack of SPA2 function in the light. We found that the SPA2 protein is inherently incapable of repressing photomorphogenesis in light-grown seedlings. The data therefore indicate that light inactivates the function of SPA2 through a post-translational mechanism that eliminates the activity of the remaining SPA2 protein in the cell.
authors.affiliation: Botanical Institute, University of Cologne, Cologne Biocenter, Zülpicher Straße 47b, 50674 Köln, Germany.
authors.forename: Martin
authors.initials: M
authors.lastname: Balcerowicz
authors.forename: Kirsten
authors.initials: K
authors.lastname: Fittinghoff
authors.forename: Lennart
authors.initials: L
authors.lastname: Wirthmueller
authors.forename: Alexander
authors.initials: A
authors.lastname: Maier
authors.forename: Petra
authors.initials: P
authors.lastname: Fackendahl
authors.forename: Gabriele
authors.initials: G
authors.lastname: Fiene
authors.forename: Csaba
authors.initials: C
authors.lastname: Koncz
authors.forename: Ute
authors.initials: U
authors.lastname: Hoecker
mesh_terms: D017360:Arabidopsis / Q000235:genetics* / Q000378:metabolism / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism* / Q000528:radiation effects; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism / Q000528:radiation effects; D035264:Flowers / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D018506:Gene Expression Regulation, Plant; D008027:Light; D018515:Plant Leaves / Q000235:genetics / Q000254:growth & development / Q000378:metabolism; D030821:Plants, Genetically Modified / Q000235:genetics / Q000378:metabolism / Q000528:radiation effects; D011401:Promoter Regions, Genetic; D011494:Protein Kinases / Q000235:genetics / Q000378:metabolism* / Q000528:radiation effects; D011499:Protein Processing, Post-Translational; D017346:Protein Serine-Threonine Kinases; D018749:RNA, Plant / Q000235:genetics; D036226:Seedlings / Q000235:genetics / Q000378:metabolism / Q000528:radiation effects*
chemical_list: D029681:Arabidopsis Proteins; D018797:Cell Cycle Proteins; D018749:RNA, Plant; C118571:SPA1 protein, Arabidopsis; D011494:Protein Kinases; D017346:Protein Serine-Threonine Kinases; C517423:SPA2 protein, Arabidopsis