Department of Biological Chemistry, School of Medicine, University of California Irvine, 240D Med Sci I, Irvine, CA 92697, USA.
Proc Natl Acad Sci U S A. 2010 Nov 16;107(46):19796-801. doi: 10.1073/pnas.1010648107. Epub 2010 Nov 1.
Ubiquitylation of proteins can be a signal for a variety of cellular processes beyond the classical role in proteolysis. The different signaling functions of ubiquitylation are thought to rely on ubiquitin-binding domains (UBDs). Several distinct UBD families are known, but their functions are not understood in detail, and mechanisms for interpretation and transmission of the ubiquitin signals remain to be discovered. One interesting example of the complexity of ubiquitin signaling is the Saccharomyces cerevisiae transcription factor Met4, which is regulated by a single lysine-48 linked polyubiquitin chain that can directly repress activity of Met4 or induce degradation by the proteasome. Here we show that ubiquitin signaling in Met4 is controlled by its tandem UBD regions, consisting of a previously recognized ubiquitin-interacting motif and a novel ubiquitin-binding region, which lacks homology to known UBDs. The tandem arrangement of UBDs is required to protect ubiquitylated Met4 from degradation and enables direct inactivation of Met4 by ubiquitylation. Interestingly, protection from proteasomes is a portable feature of UBDs because a fusion of the tandem UBDs to the classic proteasome substrate Sic1 stabilized Sic1 in vivo in its ubiquitylated form. Using the well-defined Sic1 in vitro ubiquitylation system we demonstrate that the tandem UBDs inhibit efficient polyubiquitin chain elongation but have no effect on initiation of ubiquitylation. Importantly, we show that the nonproteolytic regulation enabled by the tandem UBDs is critical for ensuring rapid transcriptional responses to nutritional stress, thus demonstrating an important physiological function for tandem ubiquitin-binding domains that protect ubiquitylated proteins from degradation.
蛋白质的泛素化除了在经典的蛋白水解中具有信号作用外，还可以作为各种细胞过程的信号。泛素化的不同信号功能被认为依赖于泛素结合结构域（UBDs）。虽然已经知道几种不同的 UBD 家族，但它们的功能尚未详细了解，并且泛素信号的解释和传递机制仍有待发现。泛素信号的复杂性的一个有趣例子是酿酒酵母转录因子 Met4，它受单个赖氨酸-48 连接的多泛素链的调节，该链可以直接抑制 Met4 的活性或诱导蛋白酶体降解。在这里，我们表明 Met4 中的泛素信号受其串联 UBD 区域的控制，该区域由先前识别的泛素相互作用基序和一个新的泛素结合区域组成，该区域与已知的 UBD 没有同源性。串联 UBD 区域的排列是保护泛素化 Met4 不被降解所必需的，并且使 Met4 能够通过泛素化直接失活。有趣的是，对蛋白酶体的保护是 UBD 的一个可移植特征，因为串联 UBD 与经典蛋白酶体底物 Sic1 的融合使 Sic1 在体内以其泛素化形式稳定。使用定义明确的 Sic1 体外泛素化系统，我们证明串联 UBD 抑制有效的多泛素链延伸，但对泛素化的起始没有影响。重要的是，我们表明串联 UBD 实现的非蛋白水解调控对于确保对营养胁迫的快速转录反应至关重要，从而证明了串联泛素结合结构域保护泛素化蛋白免于降解的重要生理功能。
