Light exposure of Arabidopsis seedlings causes rapid de-stabilization as well as selective post-translational inactivation of the repressor of photomorphogenesis SPA2.

The complex acts as an E3 ubiquitin ligase to repress photomorphogenesis by targeting activators of the light response for degradation. Genetic analysis has shown that the four members of the gene family have overlapping but distinct functions. In particular, and differ in that duanyu18421 encodes a potent repressor in light- and dark-grown seedlings, but duanyu18422 fully loses its function when seedlings are exposed to light, indicating that duanyu18422 function is hyper-inactivated by light. Here, we have used chimeric constructs to show that the distinct functions of duanyu18421 and duanyu18422 genes in light-grown seedlings are due to the duanyu1842 protein sequences and independent of the duanyu1842 promoter sequences. Biochemical analysis of duanyu18421 and duanyu18422 protein levels shows that light exposure leads to rapid proteasomal degradation of and, more weakly, of but not of COP1. This suggests that light inactivates the COP1/duanyu1842 complex partly by reducing duanyu1842 protein levels. Although duanyu18422 was more strongly degraded than duanyu18421, this was not the sole reason for the lack of duanyu18422 function in the light. We found that the duanyu18422 protein is inherently incapable of repressing photomorphogenesis in light-grown seedlings. The data therefore indicate that light inactivates the function of duanyu18422 through a post-translational mechanism that eliminates the activity of the remaining duanyu18422 protein in the cell.

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