title: FHY1 mediates nuclear import of the light-activated phytochrome A photoreceptor.
abstract: The phytochrome (phy) family of photoreceptors is of crucial importance throughout the life cycle of higher plants. Light-induced nuclear import is required for most phytochrome responses. Nuclear accumulation of phyA is dependent on two related proteins called FHY1 (Far-red elongated HYpocotyl 1) and FHL (FHY1 Like), with FHY1 playing the predominant function. The transcription of FHY1 and FHL are controlled by FHY3 (Far-red elongated HYpocotyl 3) and FAR1 (FAr-red impaired Response 1), a related pair of transcription factors, which thus indirectly control phyA nuclear accumulation. FHY1 and FHL preferentially interact with the light-activated form of phyA, but the mechanism by which they enable photoreceptor accumulation in the nucleus remains unsolved. Sequence comparison of numerous FHY1-related proteins indicates that only the NLS located at the N-terminus and the phyA-interaction domain located at the C-terminus are conserved. We demonstrate that these two parts of FHY1 are sufficient for FHY1 function. phyA nuclear accumulation is inhibited in the presence of high levels of FHY1 variants unable to enter the nucleus. Furthermore, nuclear accumulation of phyA becomes light- and FHY1-independent when an NLS sequence is fused to phyA, strongly suggesting that FHY1 mediates nuclear import of light-activated phyA. In accordance with this idea, FHY1 and FHY3 become functionally dispensable in seedlings expressing a constitutively nuclear version of phyA. Our data suggest that the mechanism uncovered in Arabidopsis is conserved in higher plants. Moreover, this mechanism allows us to propose a model explaining why phyA needs a specific nuclear import pathway.
authors.affiliation: Centre for Integrative Genomics, University of Lausanne, Lausanne, Switzerland.
authors.forename: Thierry
authors.initials: T
authors.lastname: Genoud
authors.forename: Fabian
authors.initials: F
authors.lastname: Schweizer
authors.forename: Anke
authors.initials: A
authors.lastname: Tscheuschler
authors.forename: Dimitry
authors.initials: D
authors.lastname: Debrieux
authors.forename: Jorge J
authors.initials: JJ
authors.lastname: Casal
authors.forename: Eberhard
authors.initials: E
authors.lastname: Schäfer
authors.forename: Andreas
authors.initials: A
authors.lastname: Hiltbrunner
authors.forename: Christian
authors.initials: C
authors.lastname: Fankhauser
mesh_terms: D021581:Active Transport, Cell Nucleus / Q000528:radiation effects; D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000737:chemistry / Q000235:genetics / Q000378:metabolism* / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002467:Cell Nucleus / Q000737:chemistry / Q000235:genetics / Q000378:metabolism* / Q000528:radiation effects*; D008027:Light; D008969:Molecular Sequence Data; D019913:Nuclear Localization Signals / Q000235:genetics / Q000378:metabolism; D045322:Photosynthetic Reaction Center Complex Proteins / Q000378:metabolism*; D010834:Phytochrome / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D051942:Phytochrome A / Q000235:genetics / Q000378:metabolism*; D017434:Protein Structure, Tertiary; D016415:Sequence Alignment; D014157:Transcription Factors / Q000235:genetics / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; C524723:FHL protein, Arabidopsis; C441269:FHY1 protein, Arabidopsis; D019913:Nuclear Localization Signals; D045322:Photosynthetic Reaction Center Complex Proteins; D051942:Phytochrome A; D014157:Transcription Factors; D010834:Phytochrome