Fig. 6. Working model for the molecular integration of UVR8 and cryptochrome signalling.

Light activation of UVR8 and cry1 leads to inhibition of the COP1-SPA E3 ubiquitin ligase complex, which releases HY5 from its repression through ubiquitination and proteasomal degradation. The transcriptional activator HY5 induces the expression of genes such as those associated with UV-B acclimation and tolerance (biosynthesis of sunscreen metabolites, DNA repair, photoprotection, etc.). HY5 also induces genes encoding distinct proteins with regulatory roles that concurrently provide feedback regulation of UVR8 and cryptochrome photoreceptor activities (note that the control of cry1 activity by UV-B through UVR8 signalling remains to be demonstrated). BIC1 and BIC2 are thought to mainly block cry1 homodimerization, whereas RUP1 and RUP2 are thought to primarily facilitate UVR8 re-dimerization (although additionally blocking UVR8 monomerization cannot be excluded).