Mechanisms of polyamine-dependent control of ornithine decarboxylase and its antizyme

多胺是一种重要的有机阳离子，具有多种细胞功能。鸟氨酸脱羧酶(ODC)是多胺生物合成中的限速酶。ODC周转是泛素非依赖性蛋白分解的典范。在哺乳动物和低等真核生物中，ODC的降解都是由高水平的多胺诱导的，因此构成了多胺生物合成的调节反馈控制。在哺乳动物中的工作揭示了一种机制，在这种机制中，多胺在编码ODC抗酶的mRNA的解码过程中诱导核糖体移码。该蛋白介导蛋白酶体对ODC的降解。我们在酿酒酵母中发现了一种ODC抗酶，称为Oaz1，由于其与哺乳动物抗酶的序列相似性很低，因此长期以来一直没有被检测到。Oaz1的合成受多胺诱导的核糖体移码控制。利用酵母模型系统，我们还发现Oaz1的水平还受泛素介导的蛋白分解的控制。多胺对Oaz1的降解有抑制作用。在这个项目中，我们想要表征多胺诱导的核糖体在OAZ1mRNA解码过程中移码的机制，并表征泛素素介导的Oaz1蛋白分解的原理以及多胺如何抑制这一过程。这些研究可能与理解包括人类在内的高等生物体中多胺生物合成的调节有关。

Polyamines are essential organic cations with a variety of cellular functions. Ornithine decarboxylase (ODC) is the rate-limiting enzyme in the biosynthesis of polyamines. ODC turnover is the paradigm of ubiquitin-independent proteolysis. Both in mammals and lower eukaryotes, ODC degradation is induced by high levels of polyamines, which thus constitutes a regulatory feedback control of polyamine biosynthesis. Work in mammals has revealed a mechanism wherein polyamines induce a ribosomal frameshifting during the decoding of an mRNA encoding ODC antizyme. This protein mediates degradation of ODC by the proteasome. We have discovered an ODC antizyme, termed Oaz1, in Saccharomyces cerevisiae that has long escaped detection due to its low but significant sequence similarity to mammalian antizymes. Oaz1 synthesis is controlled by polyamine-induced ribosomal frameshifting. Using the yeast model system we moreover discovered that Oaz1 levels are in addition controlled by ubiquitin-mediated proteolysis. Degradation of Oaz1 is inhibited by polyamines. In this project, we want to characterize the mechanisms that underlie polyamine-induced ribosomal frameshifting during decoding of OAZ1 mRNA and to characterize the principles of ubiquitinmediated proteolysis of Oaz1 and how this process is inhibited by polyamines. These studies are likely to be relevant for an understanding of the regulation of polyamine biosynthesis in higher organisms including humans as well.