title: ZEITLUPE is a circadian photoreceptor stabilized by GIGANTEA in blue light.
abstract: The circadian clock is essential for coordinating the proper phasing of many important cellular processes. Robust cycling of key clock elements is required to maintain strong circadian oscillations of these clock-controlled outputs. Rhythmic expression of the Arabidopsis thaliana F-box protein ZEITLUPE (ZTL) is necessary to sustain a normal circadian period by controlling the proteasome-dependent degradation of a central clock protein, TIMING OF CAB EXPRESSION 1 (TOC1). ZTL messenger RNA is constitutively expressed, but ZTL protein levels oscillate with a threefold change in amplitude through an unknown mechanism. Here we show that GIGANTEA (GI) is essential to establish and sustain oscillations of ZTL by a direct protein-protein interaction. GI, a large plant-specific protein with a previously undefined molecular role, stabilizes ZTL in vivo. Furthermore, the ZTL-GI interaction is strongly and specifically enhanced by blue light, through the amino-terminal flavin-binding LIGHT, OXYGEN OR VOLTAGE (LOV) domain of ZTL. Mutations within this domain greatly diminish ZTL-GI interactions, leading to strongly reduced ZTL levels. Notably, a C82A mutation in the LOV domain, implicated in the flavin-dependent photochemistry, eliminates blue-light-enhanced binding of GI to ZTL. These data establish ZTL as a blue-light photoreceptor, which facilitates its own stability through a blue-light-enhanced GI interaction. Because the regulation of GI transcription is clock-controlled, consequent GI protein cycling confers a post-translational rhythm on ZTL protein. This mechanism of establishing and sustaining robust oscillations of ZTL results in the high-amplitude TOC1 rhythms necessary for proper clock function.
authors: Department of Plant Cellular and Molecular Biology/Plant Biotechnology Center, Ohio State University, 054 Rightmire Hall, 1060 Carmack Road, Columbus, Ohio 43210, USA.
authors: Woe-Yeon
authors: WY
authors: Kim
authors: Sumire
authors: S
authors: Fujiwara
authors: Sung-Suk
authors: SS
authors: Suh
authors: Jeongsik
authors: J
authors: Kim
authors: Yumi
authors: Y
authors: Kim
authors: Linqu
authors: L
authors: Han
authors: Karine
authors: K
authors: David
authors: Joanna
authors: J
authors: Putterill
authors: Hong Gil
authors: HG
authors: Nam
authors: David E
authors: DE
authors: Somers
mesh_terms: D017360:Arabidopsis / Q000378:metabolism* / Q000528:radiation effects; D029681:Arabidopsis Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D002940:Circadian Rhythm / Q000502:physiology*; D003116:Color; D018506:Gene Expression Regulation, Plant; D008027:Light; D009154:Mutation / Q000235:genetics; D011485:Protein Binding / Q000528:radiation effects; D017434:Protein Structure, Tertiary
chemical_list: D029681:Arabidopsis Proteins; C121183:GI protein, Arabidopsis; C410503:ZTL protein, Arabidopsis