Ring finger motif of Arabidopsis thaliana COP1 defines a new class of zinc-binding domain.

The COP1 gene of Arabidopsis thaliana encodes a protein mediating the switch between the two developmental pathways utilized in light and darkness. A cysteine-rich motif identified the COP1 protein as a member of a group of regulatory proteins which share the amino acid motif Cys-X-X-Cys-loop I-Cys-X-His-X-X-Cys-X-X-Cys-loop II-Cys-X-X-Cys (ring finger). Although this new class of cysteine-rich motifs has been proposed to bind metal ions, no direct evidence supporting this has been presented. By analyzing the COP1 protein expressed in Escherichia coli, we demonstrate here that each COP1 molecule can bind up to two zinc atoms. The two zinc ions are bound with different affinities. One is tightly bound and resistant to urea and EDTA, whereas the other one is labile under those conditions. It is further shown that deletion of the ring finger motif abolishes the metal-binding capacity of COP1. We conclude that the ring finger motif constitutes a zinc-coordinating element distinct from previously characterized zinc-binding domains.