Borg Mikael, Mittag Tanja, Pawson Tony, Tyers Mike, Forman-Kay Julie D, Chan Hue Sun
Department of Biochemistry, Faculty of Medicine, University of Toronto, Toronto, ON, Canada M5S 1A8.
Proc Natl Acad Sci U S A. 2007 Jun 5;104(23):9650-5. doi: 10.1073/pnas.0702580104. Epub 2007 May 23.
Regulation of biological processes often involves phosphorylation of intrinsically disordered protein regions, thereby modulating protein interactions. Initiation of DNA replication in yeast requires elimination of the cyclin-dependent kinase inhibitor Sic1 via the SCF(Cdc4) ubiquitin ligase. Intriguingly, the substrate adapter subunit Cdc4 binds to Sic1 only after phosphorylation of a minimum of any six of the nine cyclin-dependent kinase sites on Sic1. To investigate the physical basis of this ultrasensitive interaction, we consider a mean-field statistical mechanical model for the electrostatic interactions between a single receptor site and a conformationally disordered polyvalent ligand. The formulation treats phosphorylation sites as negative contributions to the total charge of the ligand and addresses its interplay with the strength of the favorable ligand-receptor contact. Our model predicts a threshold number of phosphorylation sites for receptor-ligand binding, suggesting that ultrasensitivity in the Sic1-Cdc4 system may be driven at least in part by cumulative electrostatic interactions. This hypothesis is supported by experimental affinities of Cdc4 for Sic1 fragments with different total charges. Thus, polyelectrostatic interactions may provide a simple yet powerful framework for understanding the modulation of protein interactions by multiple phosphorylation sites in disordered protein regions.
生物过程的调控通常涉及内在无序蛋白区域的磷酸化，从而调节蛋白相互作用。酵母中DNA复制的起始需要通过SCF（Cdc4）泛素连接酶消除细胞周期蛋白依赖性激酶抑制剂Sic1。有趣的是，底物衔接子亚基Cdc4仅在Sic1上九个细胞周期蛋白依赖性激酶位点中的至少任意六个位点磷酸化后才与Sic1结合。为了研究这种超敏相互作用的物理基础，我们考虑了一个单受体位点与构象无序的多价配体之间静电相互作用的平均场统计力学模型。该公式将磷酸化位点视为对配体总电荷的负贡献，并探讨了其与有利的配体-受体接触强度之间的相互作用。我们的模型预测了受体-配体结合的磷酸化位点阈值数量，表明Sic1-Cdc4系统中的超敏性可能至少部分由累积静电相互作用驱动。这一假设得到了Cdc4对具有不同总电荷的Sic1片段的实验亲和力的支持。因此，多静电相互作用可能为理解无序蛋白区域中多个磷酸化位点对蛋白相互作用的调节提供一个简单而强大的框架。
