Abstract
UVR8 (UV RESISTANCE LOCUS 8) is a UV-B photoreceptor responsible for initiating UV-B signalling in plants. UVR8 is a homodimer in its signalling inactive form. Upon absorption of UV radiation, the protein monomerizes into its photoactivated state. In the monomeric form, UVR8 binds the E3 ubiquitin ligase COP1 (CONSTITUTIVELY PHOTOMORPHOGENIC 1), triggering subsequent UV-B-dependent photomorphogenic development in plants. Recent in vivo experiments have shown that the UVR8 C-terminal region (aa 397–423; UVR8C27) alone is sufficient to regulate the activity of COP1. In this work, CD spectroscopy and NMR experiments showed that the UVR8C27 domain was non-structured but gained secondary structure at higher temperatures leading to increased order. Bias-exchange metadynamics simulations were also performed to evaluate the free energy landscape of UVR8C27. An inverted free energy landscape was revealed, with a disordered structure in the global energy minimum. Flanking the global energy minimum, more structured states were found at higher energies. Furthermore, stabilization of the low energy disordered state was attributed to a proline residue, P411, as evident from P411A mutant data. P411 is also a key residue in UVR8 binding to COP1. UVR8C27 is therefore structurally competent to function as a molecular switch for interaction of UVR8 with different binding partners since at higher free energies different structural conformations are being induced in this peptide. P411 has a key role for this function.
Highlights
UV-B (280–315 nm) is only a small fraction of the total solar radiation at the earth’s surface, it has a significant impact on life on our planet
In our previous theoretical work, we predicted that the UVR8C27 domain is rather flexible and that the two residues V410 and P411 play a key role in the formation of the heterodimeric complex with COP119
These sequential monomerization/dimerization events can most likely be explained by the conformation/availability of the UV RESISTANCE LOCUS 8 (UVR8) C-terminus for binding to the different interaction partners (COP1, RUP1/2, or a second UVR8), and how the UVR8C27 domain is structurally rearranged during these different states
Summary
UV-B (280–315 nm) is only a small fraction of the total solar radiation at the earth’s surface, it has a significant impact on life on our planet. After absorption of UV quanta and monomerization of UVR8, UV-B signalling is further transmitted by the activated UVR8 monomer through its direct interaction with COP13. This is accomplished through two of its domains, a part of the C-terminal domain encompassing residues 397–423 (hereafter denoted UVR8C27) and the central RCC1 domain where the UVR8C27 domain previously has been shown to be essential for the interaction with COP113,17. In a recent study by Yin et al.[17], the UVR8C27 domain was shown to be sufficient on its own in mimicking the action of the full size UVR8 monomer with regards to its action on COP1, i.e. being able to modulate the activity of COP1 and downstream signalling in vivo, including HY5 expression. In our previous theoretical work, we predicted that the UVR8C27 domain is rather flexible and that the two residues V410 and P411 play a key role in the formation of the heterodimeric complex with COP119
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