SPA Proteins Affect the Subcellular Localization of COP1 in the COP1/SPA Ubiquitin Ligase Complex during Photomorphogenesis

Abstract

The Arabidopsis () COP1/SPA ubiquitin ligase is a central repressor that suppresses light signaling in darkness by targeting positive regulators of the light response, mainly transcription factors, for degradation. Light inactivates COP1/SPA, in part by excluding COP1 from the nucleus. SPA proteins are essential cofactors of COP1, but their exact role in the COP1/SPA complex is thus far unknown. To unravel a potential role of SPA proteins in COP1 nucleocytoplasmic partitioning, we monitored the subcellular localization of COP1 in a quadruple mutant (). We analyzed a YFP-COP1-expressing transgenic line and endogenous COP1 after subcellular fractionation. In dark-grown seedlings, both YFP-COP1 and endogenous COP1 accumulated in the nucleus in the absence and presence of SPA proteins, indicating that SPA proteins are not required for nuclear localization of COP1 in darkness. In contrast, in white light-grown seedlings, mutants failed to relocalize COP1 from the nucleus to the cytoplasm. Hence, SPA proteins are necessary for the light-controlled change in COP1 subcellular localization. We conclude that SPA proteins have a dual role: (1) they are required for light-responsiveness of COP1 subcellular localization, and (2) they promote COP1 activity in darkness in a fashion that is independent of the nuclear import/nuclear retention of COP1.

The pseudokinase TRIB 1 toggles an intramolecular switch to regulate COP 1 nuclear export

“…Like mammalian COP1, Arabidopsis COP1 has a cytoplasmic localization signal within its coiled coil domain that is necessary for its nuclear export and resembles the leucine-rich NESs recognized by CRM1 Subramanian et al, 2004). In plants, binding partners, such as SPA proteins and CSN1, that have been reported to modulate COP1 localization, associate with the putative coiled coil domain of COP1 (Wang et al, 2009;Balcerowicz et al, 2017). In plants, binding partners, such as SPA proteins and CSN1, that have been reported to modulate COP1 localization, associate with the putative coiled coil domain of COP1 (Wang et al, 2009;Balcerowicz et al, 2017).…”

“…The export of mammalian COP1 is primarily regulated by association with CRM1, as well as by 14-3-3r under conditions of cell stress in response to DNA damage (Yi et al, 2002;Su et al, 2010;Kırlı et al, 2015). In plants, binding partners, such as SPA proteins and CSN1, that have been reported to modulate COP1 localization, associate with the putative coiled coil domain of COP1 (Wang et al, 2009;Balcerowicz et al, 2017). For Arabidopsis COP1, control of COP1 subcellular localization seems to be exerted solely through interactions mediated by the putative coiled coil domain, and the WD40 domain is not essential for lightregulated nucleocytoplasmic trafficking (Torii et al, 1998;.…”

CONSTITUTIVELY PHOTOMORPHOGENIC1 promotes ABA‐mediated inhibition of post‐germination seedling establishment

“…Photoreceptors differentially regulate ABA-mediated inhibition of seedling establishment COP1 is predominantly nuclear during darkness, whereas light reduces its activity by inducing its translocation from nucleus to cytosol (von Arnim and Deng, 1994). Red, farred and blue light have a differential impact on the nuclear accumulation of COP1 (Osterlund and Deng, 1998;Balcerowicz et al, 2017). In addition to its localization, the activity of COP1 is also determined by its interaction partner SUPPRESSOR OF PHYA-105 (SPA) (Hoecker, 2017;Podolec and Ulm, 2018).…”

Phytochrome B Requires PIF Degradation and Sequestration to Induce Light Responses across a Wide Range of Light Conditions

“…These phy nuclear bodies are thought to be sites of PIF degradation and transcriptional regulation (Van Buskirk et al, 2012;Kaiserli et al, 2015) Phy promotes light responses via multiple pathways. In one pathway, phy interacts with and inhibits the function of the CON-STITUTIVELY PHOTOMORPHOGENIC1 (COP1)/SUPPRESSOR OF phyA-105 (SPA) ubiquitin E3 ligase complex by either excluding COP1 from the nucleus, by disrupting the interaction of COP1 with SPAs, or by promoting the degradation of SPA1 and SPA2 (Osterlund and Deng, 1998;Pacín et al, 2014;Chen et al, 2015;Lu et al, 2015;Sheerin et al, 2015;Balcerowicz et al, 2017). In the light, the inhibition of the COP1/SPA complex by phy causes substrate transcription factors such as ELONGATED HYPOCOTYL5 (HY5), LONG HYPOCOTYL IN FAR-RED (HFR1), and the B-BOX ZINC FINGER PROTEINs accumulate (Osterlund et al, 2000;Jang et al, 2005;Yang et al, 2005;Datta et al, 2008;Chang et al, 2011;Fan et al, 2012;Xu et al, 2016).…”