Center for Colon Cancer Research, University of South Carolina, Columbia, SC 29208, USA.
Biosci Rep. 2013 Jan 18;33(1):165-73. doi: 10.1042/BSR20120112.
TS (thymidylate synthase) is a key enzyme in the de novo biosynthesis of dTMP, and is indispensable for DNA replication. Previous studies have shown that intracellular degradation of the human enzyme [hTS (human thymidylate synthase)] is mediated by the 26S proteasome, and occurs in a ubiquitin-independent manner. Degradation of hTS is governed by a degron that is located at the polypeptide's N-terminus that is capable of promoting the destabilization of heterologous proteins to which it is attached. The hTS degron is bipartite, consisting of two subdomains: an IDR (intrinsically disordered region) that is highly divergent among mammalian species, followed by a conserved amphipathic α-helix (designated hA). In the present report, we have characterized the structure and function of the hTS degron in more detail. We have conducted a bioinformatic analysis of interspecies sequence variation exhibited by the IDR, and find that its hypervariability is not due to diversifying (or positive) selection; rather, it has been subjected to purifying (or negative) selection, although the intensity of such selection is relaxed or weakened compared with that exerted on the rest of the molecule. In addition, we have verified that both subdomains of the hTS degron are required for full activity. Furthermore, their co-operation does not necessitate that they are juxtaposed, but is maintained when they are physically separated. Finally, we have identified a 'cryptic' degron at the C-terminus of hTS, which is activated by the N-terminal degron and appears to function only under certain circumstances; its role in TS metabolism is not known.
TS（胸苷酸合成酶）是从头合成 dTMP 的关键酶，对 DNA 复制不可或缺。先前的研究表明，人类酶 [hTS（人胸苷酸合成酶）] 的细胞内降解是由 26S 蛋白酶体介导的，并且以非泛素依赖的方式发生。hTS 的降解受位于多肽 N 端的降解基序控制，该基序能够促进与其相连的异源蛋白的不稳定。hTS 降解基序是二部分的，由两个亚结构域组成：一个在哺乳动物物种中高度不同的 IDR（无规卷曲区域），随后是一个保守的两亲性α-螺旋（命名为 hA）。在本报告中，我们更详细地描述了 hTS 降解基序的结构和功能。我们对 IDR 表现出的种间序列变异进行了生物信息学分析，发现其高变异性不是由于多样化（或正）选择所致；相反，它受到了纯化（或负）选择的影响，尽管与分子的其余部分相比，这种选择的强度被放松或减弱。此外，我们已经验证了 hTS 降解基序的两个亚结构域都需要充分发挥活性。此外，它们的合作并不需要它们并置，而是在它们物理分离时保持。最后，我们在 hTS 的 C 端鉴定了一个“隐藏”的降解基序，该基序被 N 端降解基序激活，并且似乎仅在某些情况下起作用；其在 TS 代谢中的作用尚不清楚。
