Ubiquitin-conjugated degradation of golden 2-like transcription factor is mediated by CUL4-DDB1-based E3 ligase complex in tomato

CULLIN4-RING ubiquitin ligases (CRL4s) as well as their targets are fundamental regulators functioning in many key developmental and stress responses in eukaryotes. In tomato (Solanum lycopersicum), molecular cloning has revealed that the underlying genes of natural spontaneous mutations high pigment 1 (hp1), high pigment 2 (hp2) and uniform ripening (u) encode UV-DAMAGED DNA BINDING PROTEIN 1 (DDB1), DE-ETIOLATED 1 (DET1) and GOLDEN 2-LIKE (GLK2), respectively. However, the molecular basis of the opposite actions of tomato GLK2 vs CUL4-DDB1-DET1 complex on regulating plastid level and fruit quality remains unknown. Here, we provide molecular evidence showing that the tomato GLK2 protein is a substrate of the CUL4-DDB1-DET1 ubiquitin ligase complex for the proteasome degradation. SlGLK2 is degraded by the ubiquitin-proteasome system, which is mainly determined by two lysine residues (K11 and K253). SlGLK2 associates with the CUL4-DDB1-DET1 E3 complex in plant cells. Genetically impairing CUL4, DDB1 or DET1 results in a retardation of SlGLK2 degradation by the 26S proteasome. These findings are relevant to the potential of nutrient accumulation in tomato fruit by mediating the plastid level and contribute to a deeper understanding of an important regulatory loop, linking protein turnover to gene regulation.