HFR1 is targeted by COP1 E3 ligase for post-translational proteolysis during phytochrome A signaling

Upon activation by far-red light, phytochrome A signals are transduced through several pathways to promote photomorphogenesis. The COP1 E3 ligase represses photomorphogenesis in part by targeting transcription activators such as LAF1 and HY5 for destruction. Another positive regulator of photomorphogenesis is HFR1, a basic helix-loop-helix (bHLH) transcription factor. Here, we show that HFR1 colocalizes with COP1 in nuclear bodies, and that the HFR1 N-terminal region (amino acids 1-131) interacts with the COP1 WD40 domain. HFR1(DeltaN), an HFR1 mutant lacking the two N-terminal, COP1-interacting motifs, still localizes in nuclear bodies and retains weak affinity for COP1. Both HFR1 and HFR1(DeltaN) can be ubiquitinated by COP1, although with different efficiencies. Expression of 35S-HFR1(DeltaN) in wild-type plants confers greater hypersensitivity to FR than 35S-HFR1 expression, and only seedlings expressing 35S-HFR1(DeltaN) display constitutive photomorphogenesis. These phenotypic differences can be attributed to the instability of HFR1 compared with HFR1(DeltaN). In transgenic plants, HFR1 levels are significantly elevated upon induced expression of a dominant-negative COP1 mutant that interferes with endogenous COP1 E3 activity. Moreover, induced expression of wild-type COP1 in transgenic plants accelerates post-translational degradation of HFR1 under FR light. Taken together, our results show that HFR1 is ubiquitinated by COP1 E3 ligase and marked for post-translational degradation during photomorphogenesis.