Functional comparison of the WD‐repeat domains of SPA1 and COP1 in suppression of photomorphogenesis

Abstract

The Arabidopsis COP1/SPA complex acts as a cullin4‐based E3 ubiquitin ligase to suppress photomorphogenesis in darkness. It is a tetrameric complex of two COP1 and two SPA proteins. Both COP1 and SPA are essential for the activity of this complex, and they both contain a C‐terminal WD‐repeat domain responsible for substrate recruitment and binding of DDB1. Here, we used a WD domain swap‐approach to address the cooperativity of COP1 and SPA proteins. We found that expression of a chimeric COP1 carrying the WD‐repeat domain of SPA1 mostly complemented the cop1‐4‐mutant phenotype in darkness, indicating that the WD repeat of SPA1 can replace the WD repeat of COP1. In the light, SPA1‐WD partially substituted for COP1‐WD. In contrast, expression of a chimeric SPA1 protein carrying the WD repeat of COP1 did not rescue the spa‐mutant phenotype. Together, our findings demonstrate that a SPA1‐type WD repeat is essential for COP1/SPA activity, while a COP1‐type WD is in part dispensible. Moreover, a complex with four SPA1‐WDs is more active than a complex with only two SPA1‐WDs. A homology model of SPA1‐WD based on the crystal structure of COP1‐WD uncovered two insertions and several amino acid substitutions at the predicted substrate‐binding pocket of SPA1‐WD.

Dissecting the functions of COP1 in the UVR8 pathway with a COP1 variant in Arabidopsis

“…In addition to regulating development at the seedling stage, COP1 also regulates plant architecture at the adult stage. The cop1‐4 mutant exhibited a short stature as 40‐day‐old and 70‐day‐old plants (Figure S5), which was similar to previous observations (Ang & Deng, 1994; Kerner et al., 2021; Liu et al., 2008). Whereas YFP‐COP1 largely rescued the adult cop1‐4 mutant phenotypes, YFP‐COP1 C509S only partially rescued the plant height defect of the cop1‐4 mutant (Figure S5a–d).…”

Phytochrome B photobodies are comprised of phytochrome B and its primary and secondary interacting proteins

“…Among the 23 final candidates, five proteins (PCH1, PCHL, COP1, SPA1, and TZP) had previously been identified as phyB photobody components 15,16,23,[35][36][37]44 . The other candidate proteins had not previously been experimentally assessed for photobody localization.…”

Signaling Mechanisms by Arabidopsis Cryptochromes

“…The WD repeats also act in binding DDB1 in the Cullin 4-complex ( Ponnu and Hoecker, 2021 ). Since the WD-repeat of SPA1 can replace that of COP1 but not vice versa, their functions are likely related but not identical ( Kerner et al, 2021 ). COP1/SPA complex formation is conferred by the coiled-coil domains present in COP1 and SPA proteins.…”