Photoactivation and inactivation of Arabidopsis cryptochrome 2

Abstract

Cryptochromes are blue-light receptors that regulate development and the circadian clock in plants and animals. We found that Arabidopsis cryptochrome 2 (CRY2) undergoes blue light–dependent homodimerization to become physiologically active. We identified BIC1 (blue-light inhibitor of cryptochromes 1) as an inhibitor of plant cryptochromes that binds to CRY2 to suppress the blue light–dependent dimerization, photobody formation, phosphorylation, degradation, and physiological activities of CRY2. We hypothesize that regulated dimerization governs homeostasis of the active cryptochromes in plants and other evolutionary lineages.

Coordination of Cryptochrome and Phytochrome Signals in the Regulation of Plant Light Responses

“…Homodimerization is required for the phosphorylation and physiological activity of the crys [50][51][52]. Recently, it was found that cry2 forms a homodimer in response to B light [53]. Thus, these results indicate that B light-dependent homodimerization is the elementary process of cry2 photoactivation.…”

“…The B light inhibitors of cryptochromes (BIC) 1 and 2 were identified as negative regulators of cry1 and cry2, and they interact with cry2 to suppress the B light-dependent dimerization, phosphorylation, and physiological activities. Therefore, BIC1 and BIC2 constitute the desensitizing mechanism of both crys to sustain the homeostasis of physiologically active cry [53]. However, whether BICs inhibit cry1 dimer formation remains to be tested.…”

“…Although cry1 localizes equally in the nucleus and cytoplasm [58,59], the nuclear-targeted cry1 regulates most cry1-mediated responses [58]. In addition, thousands of genes are regulated by both cry1 and cry2 in response to B light during the de-etiolation process [53,[60][61][62][63]. Thus, it is generally accepted that photoactivated cry1 and cry2 transduce light signals to downstream components, primarily in the nucleus.…”

Beyond the photocycle — how cryptochromes regulate photoresponses in plants?

“…These results demonstrate that homodimerization or oligomerization is necessary for the function of plant cryptochromes. Although no obvious blue light dependence was detected for the CRY1 or CRY2 dimerization in those earlier experiments [32 •• ], the blue light-dependent homodimerization, also referred to as photodimerization, of Arabidopsis CRY1 and CRY2 has been recently detected in human HEK293 cells and in transgenic plants expressing near stoichiometric amounts of two recombinant CRY proteins fused to different epitope tags [36 •• ,37] (Q. Wang, unpublished results). Cryptochrome photodimerization is apparently a regulated process in plant cells.…”

“…Cryptochrome photodimerization is apparently a regulated process in plant cells. Two closely related CRY inhibitory proteins, referred to as BIC1 and BIC2 (Blue-light Inhibitor of Cryptochromes 1 and 2), were identified in a genetic screen to search for negative regulators of cryptochromes [36 •• ]. BICs inhibit blue light-dependent CRY2 homo-dimerization, oligomerization, and photobody formation.…”