Streaker Emily D, Beckett Dorothy
Department of Chemistry and Biochemistry and Center for Biological Structure and Organization, University of Maryland, College Park, Maryland 20742, USA.
Biochemistry. 2006 May 23;45(20):6417-25. doi: 10.1021/bi052599r.
An organism's response to environmental and metabolic cues requires communication between transcription regulatory processes and "other" cellular events. In a number of biological control circuits, the communication is carried out by a single multifunctional protein that participates directly in transcription initiation and in at least one other cellular process. Structural studies suggest that the function of these proteins is dictated by the formation of mutually exclusive protein-protein interactions. However, the rules that govern partner, and thus functional switching, are not known. In the Escherichia coli Biotin Regulatory System, the bifunctional protein, BirA, catalyzes post-translational biotin addition to a biotin-dependent carboxylase and binds sequence-specifically to DNA to repress transcription initiation at the biotin biosynthetic operon. Previous structural and modeling studies suggest that BirA function is determined by formation of alternative homo- and heterodimeric protein-protein interactions. In this work, the BirA functional switch is investigated using DNaseI footprinting and MALDI-TOF mass spectrometry. Results of these measurements indicate that BirA can be selectively targeted toward its enzymatic function simply by increasing the kinetic probability of heterodimerization relative to that of homodimerization. Subsequent shifting to the DNA binding function occurs as the pool of heterodimer partner is depleted and homodimerization dominates. The data support a switching mechanism in which BirA's function is dictated by its probability of encountering a particular protein partner.
生物体对环境和代谢信号的反应需要转录调控过程与“其他”细胞事件之间进行通信。在许多生物控制回路中，这种通信是由一种单一的多功能蛋白质来完成的，该蛋白质直接参与转录起始和至少一种其他细胞过程。结构研究表明，这些蛋白质的功能由相互排斥的蛋白质-蛋白质相互作用的形成所决定。然而，支配伴侣以及功能转换的规则尚不清楚。在大肠杆菌生物素调节系统中，双功能蛋白BirA催化将生物素翻译后添加到生物素依赖性羧化酶上，并序列特异性地结合到DNA上以抑制生物素生物合成操纵子处的转录起始。先前的结构和建模研究表明，BirA的功能由替代性同源和异源二聚体蛋白质-蛋白质相互作用的形成所决定。在这项工作中，使用DNaseI足迹法和MALDI-TOF质谱法研究了BirA功能开关。这些测量结果表明，只需通过增加异源二聚化相对于同源二聚化的动力学概率，BirA就可以选择性地靶向其酶促功能。随着异源二聚体伴侣库的耗尽且同源二聚化占主导地位，随后会转变为DNA结合功能。数据支持一种切换机制，其中BirA的功能由其遇到特定蛋白质伴侣的概率所决定。
