Crystal Structure Of TCP Domain Of OsPCF6 In Oryza Sative And Its DNA Binding Research

TCP family is a kind of plant-specific transcription factors that widely exists in various species.TCP proteins play key roles in a multiplicity of pathways related to plant architecture,cell proliferation and hormone regulation.These proteins contain a highly conserved TCP domain consist of a basic region in the N-terminus followed by a Helix-Loop-Helix motif,which predicted to form a bHLH conformation.To date,the functions of TCPs are well studied,but the three-dimensional structure of TCP domain is lacking.Here,we have determined the crystal structure of TCP domain from OsPCF6 through X-ray crystallography.The structure reveals that the TCP domain adopts a homo-dimeric ribbon-helix-helix(RHH)arrangement distinct from the predicted canonical basic helix-loop-helix(bHLH)structure.The basic region of TCP domain forms β-strand while the α-helix in bHLH domain,as well as the angle between the helices in the HLH motif is quite different.Interestingly,although the basic region of TCP dimer forms a three short interrupted antiparallel β-sheet arrangement instead of a common long antiparallel β-sheet in the RHH family,the whole TCP shares high topological similarity with the RHH proteins despite no sequence similarity.Thus,the structure information suggested that TCPs should belong to the RHH family.Based on the TCP-DNA complex model by docking a DNA segment into the surface β-sheet,site-directed mutagenesis and isothermal titration calorimetry(ITC)assays indicate residues R16,R17 and R19 are the key residues involved in the DNA binding.Our work will contribute to an in-depth investigation and well understanding of the mechanisms for the functional diversity of TCP family.