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

The Arabidopsis (Arabidopsis thaliana) 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 in part by excluding COP1 from the nucleus. proteins are essential cofactors of COP1, but their exact role in the COP1/duanyu1842 complex is thus far unknown. To unravel a potential role of duanyu1842 proteins in COP1 nucleocytoplasmic partitioning, we monitored the subcellular localization of COP1 in a spa1234 quadruple mutant (spaQn). 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 duanyu1842 proteins, indicating that duanyu1842 proteins are not required for nuclear localization of COP1 in darkness. In contrast, in white light-grown seedlings, spaQn mutants failed to relocalize COP1 from the nucleus to the cytoplasm. Hence, duanyu1842 proteins are necessary for the light-controlled change in COP1 subcellular localization. We conclude that duanyu1842 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.

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