Repression of photomorphogenesis in Arabidopsis thaliana requires activity of the COP9 signalosome (CSN), CDD, and COP1 complexes, but how these three complexes work in concert to accomplish this important developmental switch has remained unknown. Here, we demonstrate that Arabidopsis CULLIN4 (CUL4) associates with the CDD complex and a common catalytic subunit to form an active E3 ubiquitin ligase both in vivo and in vitro. The partial loss of function of CUL4 resulted in a constitutive photomorphogenic phenotype with respect to morphogenesis and light-regulated gene expression. Furthermore, CUL4 exhibits a synergistic genetic interaction with COP10 and DET1. Therefore, this CUL4-based E3 ligase is essential for the repression of photomorphogenesis. This CUL4-based E3 ligase appears to associate physically with COP1 E3 ligase and positively regulates the COP1-dependent degradation of photomorphogenesis-promoting transcription factors, whereas the CSN controls the biochemical modification of CUL4 essential for E3 activity. Thus, this study suggests a biochemical activity connection between CSN and CDD complexes in their cooperation with COP1 in orchestrating the repression of photomorphogenesis.

Plant phototropism is an adaptive response to changes in light direction, quantity, and quality that results in optimization of photosynthetic light harvesting, as well as water and nutrient acquisition. Though several components of the phototropic signal response pathway have been identified in recent years, including the blue light (BL) receptors phototropin1 (phot1) and phot2, much remains unknown. Here, we show that the phot1-interacting protein NONPHOTOTROPIC HYPOCOTYL3 (NPH3) functions as a substrate adapter in a CULLIN3-based E3 ubiquitin ligase, CRL3 NPH3 . Under low-intensity BL, CRL3 NPH3 mediates the mono/multiubiquitination of phot1, likely marking it for clathrin-dependent internalization from the plasma membrane. In high-intensity BL, phot1 is both mono/multi-and polyubiquitinated by CRL3 NPH3 , with the latter event targeting phot1 for 26S proteasome-mediated degradation. Polyubiquitination and subsequent degradation of phot1 under high-intensity BL likely represent means of receptor desensitization, while mono/multiubiquitination-stimulated internalization of phot1 may be coupled to BL-induced relocalization of hormone (auxin) transporters.

The target of rapamycin (TOR) plays a central role in eukaryotic cell growth control1. With prevalent hyper-activation of the mTOR pathway in human cancers2, novel strategies to enhance TOR pathway inhibition are highly desirable. We used a yeast-based platform to identify small-molecule enhancers of rapamycin (SMERs) and discovered an inhibitor of the SCFMet30 ubiquitin ligase (SMER3). The large SCF (Skp1-Cullin-F-box) family of ubiquitin ligases performs important functions in diverse cellular processes including transcription, cell-cycle control, and immune response3. Accordingly, there would be great value in developing SCF ligase inhibitors that act by a defined mechanism to specifically inactivate ligase activity. We show here that SMER3 selectively inhibits SCFMet30 in vivo and in vitro, but not the closely related SCFCdc4. Our results demonstrate that there is no fundamental barrier to obtaining specific inhibitors to modulate function of individual SCF complexes, and suggest new strategies for combination therapy with rapamycin.