Identification of a structural motif that confers specific interaction with the WD40 repeat domain of Arabidopsis COP1

Arabidopsis COP1 is a photomorphogenesis repressor capable of directly inte racting with the photomorphogenesis-promoting factor HY5, This interaction between HY5 and COP1 results in targeted degradation of HY5 by the 26S prot easome. Here we characterized the WD40 repeat domain-mediated interactions of COP1 with HY5 and two new proteins. Mutational analysis of those interac tive partners revealed a conserved motif responsible for the interaction wi th the WD40 domain. This novel motif, with the core sequence V-P-E/D-phi -G (phi = hydrophobic residue) in conjunction with an upstream stretch of 4-5 negatively charged residues, interacts with a defined surface area of the P-propeller assembly of the COP1 WD40 repeat domain through both hydrophobi c and ionic interactions. Several residues in the COP1 WD40 domain that are critical for the interaction with this motif have been revealed. The fact that point mutations either in the COP1 WD40 domain or in the HY5 motif tha t abolish the interaction between COP1 and HY5 in yeast result in a dramati c reduction of HY5 degradation in transgenic plants validates the biologica l significance of this defined interaction.