The crystal structure of the TCP domain of PCF6 in Oryza sativa L. reveals an RHH-like fold.

The TCP domain is an evolutionarily conserved DNA binding domain unique to the plant kingdom. To date, the functions of TCPs have been well studied, but the three-dimensional structure of the TCP domain is lacking. Here, we have determined the crystal structure of the TCP domain from OsPCF6. The structure reveals that the TCP domain adopts three short β strands followed by a helix-loop-helix structure, distinct from the canonical basic helix-loop-helix (bHLH) structure. This folded domain shows high structural similarity to the ribbon-helix-helix (RHH) transcriptional repressors, a family of DNA-binding proteins with a conserved 3D structural motif (RHH fold), indicating that TCPs could be reclassified as RHH proteins. Our work will provide insight towards a better understanding of the mechanisms underlying TCP protein function.