The cryptochrome and photolyase families consist of structurally related flavin adenine dinucleotide (FAD) proteins that use the absorption of blue light to accomplish different tasks. The photolyasess use the blue light for light-driven electron transfer to repair UV-damaged DNA, while the cryptochromes are blue-light photoreceptors involved in the circadian clock for plants and animals [, ].Members of this subfamily are from plants; they appear mostly to be regulatory proteins that respond to blue light. For instance, Arabidopsis cryptochromes Cry1 and Cry2 antagonistically regulate primary root elongation [, ]. Cry2 is also reported to interact with CIB1 and regulate transcription and floral initiation [].
BIC1 (blue-light inhibitor of cryptochromes 1) and homologue BIC2 suppress the functions of plant cryptochromes. They are thought to act redundantly to inhibit the function of CRYs. BIC1 has be shown to suppresss blue light-dependent CRY2 dimerization and inhibit CRY2-dependent photoresponses of plants [].
In Arabidopsis, SPA1/2/3/4 play a central role in suppression of photomorphogenesis. SPA1 and SPA2 predominate in dark-grown seedlings, whereas SPA3 and SPA4 prevalently regulate the elongation growth in adult plants []. SPAs contain a kinase-like domain, a coiled-coil domain and the WD-repeats. SPAs and COP1 (a ring finger E3 ubiquitin ligase) can form homo- and heterodimers via their respective coiled-coil domains, and the COP1/SPA complex forms a tetramer of two COP1 and two SPA proteins []. The SPA proteins can self-associate or interact with each other, forming a heterogeneous group of SPA-COP1 complexes []. Besides recognizing substrates, both COP1 and SPA bind DDB1 in the CUL4 complex through their C-terminal WD-repeat domains. They serve as DDB1-CUL4-associated factors (DCAFs) similar to other substrate adaptors in CUL4-based E3 ligases. SPA1 interacts with photoreceptor cry2 via its kinase-like domain, with cry1 via its WD-repeat domain and with phytochromes possibly via both []. SPAs have also been shown to regulate the phyB-PIF4 module at high ambient temperature [].
