Photoactivated UVR8-COP1 Module Determines Photomorphogenic UV-B Signaling Output in Arabidopsis

In arabidopsis,ultraviolet (uv)-b-induced photomorphogenesis is initiated by a unique photoreceptor uv resistance locus 8 (uvr8) which utilizes its tryptophan residues as internal chromophore to sense uv-b. as a result of uv-b light perception,the uvr8 homodimer shaped by its arginine residues undergoes a conformational switch of monomerization. then uvr8 associates with the constitutively photomorphogenic 1-suppressor of phya (cop1-spa) core complex(es) that is released from the cullin 4-damaged dna binding protein 1 (cul4-ddb1) e3 apparatus. this association,in turn,causes cop1 to convert from a repressor to a promoter of photomorphogenesis. it is not fully understood,however,regarding the biological significance of light-absorbing and dimer-stabilizing residues for uvr8 activity in photomorphogenic uv-b signaling. here,we take advantage of transgenic uvr8 variants to demonstrate that two light-absorbing tryptophans,w233 and w285,and two dimer-stabilizing arginines,r286 and r338,play pivotal roles in uv-b-induced photomorphogenesis. mutation of each residue results in alterations in uv-b light perception,uvr8 monomerization and uvr8-cop1 association in response to photomorphogenic uv-b. we also identify and functionally characterize two constitutively active uvr8 variants,uvr8w285a and uvr8r338a,whose photobiological activities are enhanced by the repression of cul4,a negative regulator in this pathway. based on our molecular and biochemical evidence,we propose that the uvr8-cop1 affinity in plants critically determines the photomorphogenic uv-b signal transduction coupling with uvr8-mediated uv-b light perception. © 2014 huang et al.