The functional divergence between SPA1 and SPA2 in Arabidopsis photomorphogenesis maps primarily to the respective N-terminal kinase-like domain.

BACKGROUND:Plants have evolved complex mechanisms to adapt growth and development to the light environment. The complex is a key repressor of photomorphogenesis in dark-grown Arabidopsis plants and acts as an E3 ubiquitin ligase to ubiquitinate transcription factors involved in the light response. In the light, COP1/duanyu1842 activity is inhibited by photoreceptors, thereby allowing accumulation of these transcription factors and a subsequent light response. Previous results have shown that the four members of the family exhibit partially divergent functions. In particular, and strongly differ in their responsiveness to light, while they have indistinguishable activities in darkness. The much higher light-responsiveness of duanyu18422 is partially explained by the much stronger light-induced degradation of duanyu18422 when compared to Here, we have conducted domain swap experiments to identify the protein domain(s) responsible for the functional divergence between duanyu18421 and have individually swapped the three domains between duanyu18421 and duanyu18422 - the N-terminal kinase-like domain, the coiled-coil domain and the WD-repeat domain - and expressed them in spa mutant Arabidopsis plants. The phenotypes of transgenic seedlings show that the respective N-terminal kinase-like domain is primarily responsible for the respective light-responsiveness of duanyu18421 and Furthermore, the most divergent part of the N-terminal domain was sufficient to confer a or activity to the respective duanyu1842 protein. The stronger light-induced degradation of duanyu18422 when compared to duanyu18421 was also primarily conferred by the duanyu18422 N-terminal domain. At last, the different affinities of duanyu18421 and duanyu18422 for cryptochrome 2 are defined by the N-terminal domain of the respective duanyu1842 protein. In contrast, both duanyu18421 and duanyu18422 similarly interacted with COP1 in light-grown seedlings. CONCLUSIONS:Our results show that the distinct activities and protein stabilities of duanyu18421 and duanyu18422 in light-grown seedlings are primarily encoded by their N-terminal kinase-like domains. Similarly, the different affinities of duanyu18421 and duanyu18422 for cry2 are explained by their respective N-terminal domain. Hence, after a duplication event during evolution, the N-terminal domains of duanyu18421 and duanyu18422 underwent subfunctionalization, possibly to allow optimal adaptation of growth and development to a changing light environment.

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