Cloning, purification and preliminary X‐ray data analysis of the human ID2 homodimer

The ID proteins are named for their role as inhibitors of DNA binding and differentiation. They contain a helix–loop–helix (HLH) domain but lack a basic DNA‐binding domain. In complex with basic HLH (bHLH) transcription factors, gene expression is regulated by DNA‐binding inactivation. Although the HLH domain is highly conserved and shares a similar topology, the IDs preferentially bind class I bHLH‐group members such as E47 (TCF3) but not the class III bHLH member Myc. A structure of an ID protein could potentially shed light on its mechanism. Owing to their short half‐lives in vivo and reported in vitro instability, this paper describes the strategies that went into expressing sufficient soluble and stable ID2 to finally obtain diffraction‐quality crystals. A 2.1 Å resolution data set was collected from a crystal belonging to space group P 3 1 21 with unit‐cell parameters a = b = 51.622, c = 111.474 Å, α = β = 90, γ = 120° that was obtained by hanging‐drop vapour diffusion in a precipitant solution consisting of 0.1  M MES pH 6.5, 2.0  M potassium acetate. The solvent content was consistent with the presence of one or two molecules in the asymmetric unit.