Stop CRYing! Inhibition of cryptochrome function by small proteins

Abstract

Plants can detect the presence of light using specialised photoreceptor proteins. These photoreceptors measure the intensity of light, but they can also respond to different spectra of light and thus 'see' different colours. Cryptochromes, which are also present in animals, are flavin-based photoreceptors that enable plants to detect blue and ultraviolet-A (UV-A) light. In Arabidopsis, there are two cryptochromes, CRYPTOCHROME 1 (CRY1) and CRYPTOCHROME 2 (CRY2) with known sensory roles. They function in various processes such as blue-light mediated inhibition of hypocotyl elongation, photoperiodic promotion of floral initiation, cotyledon expansion, anthocyanin production, and magnetoreception, to name a few. In the dark, the cryptochromes are in an inactive monomeric state and undergo photochemical and conformational change in response to illumination. This results in flavin reduction, oligomerisation, and the formation of the 'cryptochrome complexome'. Mechanisms of cryptochrome activation and signalling have been extensively studied and found to be conserved across phylogenetic lines. In this review, we will therefore focus on a far lesser-known mechanism of regulation that is unique to plant cryptochromes. This involves inhibition of cryptochrome activity by small proteins that prevent its dimerisation in response to light. The resulting inhibition of function cause profound alterations in economically important traits such as plant growth, flowering, and fruit production. This review will describe the known mechanisms of cryptochrome activation and signalling in the context of their modulation by these endogenous and artificial small inhibitor proteins. Promising new applications for biotechnological and agricultural applications will be discussed.

Keywords: cryptochrome; light signaling; microproteins.

Figure 1.. Control of cryptochromes by BICs.

(A) In response to blue light (BL), the CRY monomers undergo photochemical reduction in the flavin cofactor (FAD), followed by dimerisation and formation of an active dimer. (B) Domain overview of cryptochromes and BICs. (C) The CRY PH domain can interact with the BIC CID domain, forming an inactive CRY-BIC heterodimer. This prevents the homodimerisation and subsequent activation of CRYs. (D) In wild-type plants (first panel), exposure to blue light triggers the de-etiolation process and stops the hypocotyl from elongating. The cry1/cry2 double mutant (second panel) leads to elongated hypocotyls, while the overexpression of CRY1 (third panel) results in shortened hypocotyls. The bic1/bic2 double mutant (fourth panel), which is unable to repress cryptochrome function, has a phenotype similar to the CRY1 overexpression mutant, while the overexpression of BIC1 (fifth panel) has a similar phenotype to the cry1/cry2 double mutant.

Figure 2.. Multiple sequence alignment of A. thaliana BIC1 and BIC2 CID domain with P. patens BIC homologues.

Residues highlighted in blue have been shown to abolish the binding of BIC2 to CRY2 [18].

Figure 3.. Multiple sequence alignment of A. thaliana CRY1 and CRY2 homologues.

In green and blue are all CRY2 residues that interact with BIC2 (according to [18]). Those in blue were shown by mutational studies to abolish or reduce binding of BIC2 to CRY2.

Figure 4.. Synthetic microProteins can control cryptochrome function.

(A) Domain overview of full-length A. thaliana CRY1 (top) and the synthetic CRY-microProtein that contains only the PH domain required for dimerisation. (B) The CRY-microProtein disrupts the formation of an active CRY homodimer by forming an inactive CRY-microProtein/CRY heterodimer, thereby sequestering the CRY monomers.