How Does Photoreceptor UVR8 Perceive a UV-B Signal?

Abstract

UVR8 is the only known plant photoreceptor that mediates light responses to UV-B (280-315 nm) of the solar spectrum. UVR8 perceives a UV-B signal via light-induced dimer dissociation, which triggers a wide range of cellular responses involved in photomorphogenesis and photoprotection. Two recent crystal structures of Arabidopsis thaliana UVR8 (AtUVR8) have revealed unusual clustering of UV-B-absorbing Trp pigments at the dimer interface and provided a structural framework for further mechanistic investigation. This review summarizes recent advances in spectroscopic, computational and crystallographic studies on UVR8 that are directed toward full understanding of UV-B perception at the molecular level.

Figure 1

Absorption spectra and chromophores of red-light (in red), blue-light (in blue) and UV-B (in magenta) photoreceptors. The gray curve represents the UV/visible region of the solar spectrum measured on the surface of the earth. In red-light photoreceptor phytochromes, the bilin chromophore is covalently attached to a cysteine residue via a thioether linkage.

Figure 2

UVR8-mediated light signaling pathway in Arabidopsis. UV-B signaling begins with light-induced dimer dissociation. Monomeric UVR8 directly interacts with E3 ubiquitin ligase COP1 to regulate transcription of genes involved in UV-B responses. In contrast to phytochromes (PHYs) and cryptochromes (CRYs), UVR8 acts as a positive regulator of the COP1 functions. RUP1/RUP2 are proteins that facilitate re-dimerization of UVR8.

Figure 3

Crystal structure of the AtUVR8 core domain (PDB ID: 4D9S. a) The AtUVR8 dimer consists of two 7-blade β-propeller structures that are roughly related by 2-fold symmetry axis, which is marked in two orthogonal views by a red oval shape (left) and an arrow (right), respectively. The rainbow color (from blue to red) of the ribbon diagram represents the topological order from the N- to C-terminus. b) A top-down view highlights aromatic (Trp/Tyr/Phe; yellow), positively-charged (Arg/Lys; blue) and negatively-charged (Asp/Glu; red) residues at the dimer interface. The 7-blade propeller structure is outlined by red dashed line. c) Distribution of UV-B-absorbing Trp pigments (interfacial Trps in yellow; distal Trps in magenta) in the dimer scaffold. d) Structural alignment of UVR8 (gray) and RCC1 (rainbow coloring from blue to red corresponds to residue numbering from the N- to C-terminus). The hybrid blade 1 of RCC1 consists of strands in blue and red from both termini (right panel).

Figure 4

Characterization of the UVR8 photo-activity. a) Absorption spectra of AtUVR8 (WT: black; W285F: gray); b) Denaturing polyacrylamide gel electrophoresis (SDS-PAGE) of boiled and unboiled AtUVR8 samples before and after UV-B irradiation; c) Size-exclusive chromatography (SEC) of the AtUVR8 samples before (red) and after (green) UV-B irradiation; d) Circular dichroism spectroscopy (CD) in the far-UV region; e) Fluorescence spectroscopy of tryptophans shows different time courses of fluorescence emission for the AtUVR8 samples before (red) and after (green) UV-B irradiation.

Figure 5

Molecular mechanism of UVR8 photo-dissociation. a) Cation-π interactions between Trp (yellow) and Arg/Lys (blue) residues at the dimer interface; b) Light-induced charge separation in the Trp triad generate a large dipole moment. Subsequent photon/electron transfer leads to charge neutralization of inter-subunit salt bridges. c) Light-induced structural changes originate at two equivalent epicenters at the dimer interface. Difference electron densities observed by dynamic crystallography revealed atomic displacements associated with negative (red) to positive (green) densities. d) Motions in Trp233/Trp285 lead to ejection of the epicenter water (red sphere) thereby disrupting hydrogen bonds and salt bridges at the dimer interface. e) A sequence of light-induced structural events leading up to dimer dissociation. Note: panel c) d) e) are reproductions of Fig. 1a, Fig. 4b and Fig. 4c from reference [27].

Figure 6

Structural features beyond the UVR8 core domain. a) Sequence alignment in the N-terminal (1-13) and C-terminal (395-440) regions generated by WebLogo [55]. The size of a letter corresponds to the frequency of a specific residue at that location, i.e. sequence conservation. Distribution of charged residues reveals a negatively charged N-terminus. Secondary structure prediction indicates a charged C-terminal region with several β-strands. On the right illustrates the assembly mode of the UVR8 dimer, and the order of blades is indicated by both numbers and red arrows. The proposed structure of the N- and C-terminal regions is based on secondary structure prediction. b) Schematic illustration about possible modes of association among three interacting partners: UVR8 (heptagon with its C-terminal region in green oval), COP1 (orange) and RUP1/2 (blue). On the right shows their respective domain structures that carry a common WD40 motif suggesting similar β-propeller scaffold. Steric effect may play a role in discriminative binding of COP1 to the monomer and dimer forms of UVR8.