title: Arabidopsis cryptochrome 2 (CRY2) functions by the photoactivation mechanism distinct from the tryptophan (trp) triad-dependent photoreduction.
abstract: Cryptochromes are blue-light receptors mediating various light responses in plants and animals. The photochemical mechanism of cryptochromes is not well understood. It has been proposed that photoactivation of cryptochromes involves the blue-light-dependent photoreduction of flavin adenine dinucleotide via the electron transport chain composed of three evolutionarily conserved tryptophan residues known as the "trp triad." We investigated this hypothesis by analyzing the photochemical and physiological activities of Arabidopsis cryptochrome 2 (CRY2) mutations altered in each of the three trp-triad residues. We found that all trp-triad mutations of CRY2 tested lost photoreduction activity in vitro but retained the physiological and biochemical activities in vivo. Some of the trp-triad mutations of CRY2 remained responsive to blue light; others, such as CRY2(W374A), became constitutively active. In contrast to wild-type CRY2, which undergoes blue-light-dependent interaction with the CRY2-signaling proteins SUPPRESSOR OF PHYA 1 (SPA1) and cryptochrome-interaction basic helix-loop-helix 1 (CIB1), the constitutively active CRY2(W374A) interacts with SPA1 and CIB1 constitutively. These results support the hypothesis that cryptochromes mediate blue-light responses via a photochemistry distinct from trp-triad-dependent photoreduction and that the trp-triad residues are evolutionarily conserved in the photolyase/cryptochrome superfamily for reasons of structural integrity rather than for photochemistry per se.
authors.affiliation: Bioenergy and Biomaterial Research Center,  Hunan University, Changsha 410082,  China.
authors.forename: Xu
authors.initials: X
authors.lastname: Li
authors.forename: Qin
authors.initials: Q
authors.lastname: Wang
authors.forename: Xuhong
authors.initials: X
authors.lastname: Yu
authors.forename: Hongtao
authors.initials: H
authors.lastname: Liu
authors.forename: Huan
authors.initials: H
authors.lastname: Yang
authors.forename: Chenxi
authors.initials: C
authors.lastname: Zhao
authors.forename: Xuanming
authors.initials: X
authors.lastname: Liu
authors.forename: Chuang
authors.initials: C
authors.lastname: Tan
authors.forename: John
authors.initials: J
authors.lastname: Klejnot
authors.forename: Dongping
authors.initials: D
authors.lastname: Zhong
authors.forename: Chentao
authors.initials: C
authors.lastname: Lin
mesh_terms: D017360:Arabidopsis / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000378:metabolism; D051792:Basic Helix-Loop-Helix Proteins / Q000378:metabolism; D018797:Cell Cycle Proteins / Q000378:metabolism; D056931:Cryptochromes / Q000737:chemistry* / Q000235:genetics / Q000502:physiology; D008027:Light; D008957:Models, Genetic; D008958:Models, Molecular; D008968:Molecular Conformation; D016297:Mutagenesis, Site-Directed; D010084:Oxidation-Reduction; D010777:Photochemistry / Q000379:methods; D017434:Protein Structure, Tertiary; D014364:Tryptophan / Q000737:chemistry*
chemical_list: D029681:Arabidopsis Proteins; D051792:Basic Helix-Loop-Helix Proteins; D018797:Cell Cycle Proteins; D056931:Cryptochromes; D014364:Tryptophan; C533832:CIB1 protein, Arabidopsis; C118571:SPA1 protein, Arabidopsis