The heterotrimeric CCAAT-binding complex (CBC) is a fundamental eukaryotic transcription factor recognizing the CCAAT box. In certain fungi, like Aspergilli, the CBC cooperates with the basic leucine zipper HapX to control iron metabolism. HapX functionally depends on the CBC, and the stable interaction of both requires DNA. To study this cooperative effect, X-ray structures of the CBC-HapX-DNA complex were determined. Downstream of the CCAAT box, occupied by the CBC, a HapX dimer binds to the major groove. The leash-like N terminus of the distal HapX subunit contacts the CBC, and via a flexible polyproline type II helix mediates minor groove interactions that stimulate sequence promiscuity. In vitro and in vivo mutagenesis suggest that the structural and functional plasticity of HapX results from local asymmetry and its ability to target major and minor grooves simultaneously. The latter feature may also apply to related transcription factors such as yeast Hap4 and distinct Yap family members.

异源三聚体 CCAAT 结合复合物 (CBC) 是识别 CCAAT 盒的基本真核转录因子。在某些真菌中，如曲霉，CBC 与基本的亮氨酸拉链 HapX 合作以控制铁代谢。HapX 在功能上依赖于 CBC，两者的稳定相互作用需要 DNA。为了研究这种协同效应，确定了 CBC-HapX-DNA 复合物的 X 射线结构。在被 CBC 占据的 CCAAT 盒下游，HapX 二聚体与大沟结合。远端 HapX 亚基的皮带状 N 末端与 CBC 接触，并通过灵活的多脯氨酸 II 型螺旋介导刺激序列混杂的小沟相互作用。体外和体内诱变表明，HapX 的结构和功能可塑性源于局部不对称及其同时靶向主要和次要凹槽的能力。后一个特征也可能适用于相关的转录因子，如酵母 Hap4 和不同的 Yap 家族成员。