Species Barriers of Yeast Prions

“朊病毒”一词的意思是“感染性蛋白质”，一种无需伴随核酸即可传播疾病或性状的蛋白质。 朊病毒的概念起源于对哺乳动物传染性海绵状脑病 (TSE) 的研究，这是一组均致命的疾病，其根本原因似乎是由 PrP 蛋白组成的淀粉样蛋白的形成。 绵羊瘙痒病是一种 TSE，很容易（通过注射）传播给其他绵羊，但只有经过很长的潜伏期才传播给山羊。 随后的山羊到山羊传输显示出更短的延迟。 这就是最初的“物种障碍”，众所周知，还有更戏剧性的版本。 仓鼠瘙痒症根本不会传染给小鼠，而小鼠瘙痒症则很难传染给仓鼠。 1994 年，我们分别基于 Ure2 和 Sup35 蛋白的自我繁殖失活，描述了酿酒酵母的两种朊病毒，称为 URE3 和 PSI (1)。 Ure2p 是氮分解代谢的调节剂，URE3 菌株显示出通常受该蛋白抑制的基因的去抑制。 Sup35p 是翻译终止因子的一个亚基，PSI 菌株显示翻译终止密码子的通读率升高，其表型与sup35 突变体相似。 在每种情况下，朊病毒的形成都是大部分蛋白质转化为淀粉样蛋白形式，淀粉样蛋白是一种富含β-折叠的丝状形式，其中蛋白质无法执行其正常功能（参见参考文献2）。 我们已经证明，URE3 和 PSI 朊病毒是酵母疾病，对其生存或繁殖有害，因此尽管这两种朊病毒都具有传染性，但在 70 个野生菌株中并未发现这两种朊病毒 (3)。 相比之下，所有已知的病毒或质粒核酸感染元件都存在于野生毒株的某些部分中 (3)。 我们已经证明，Ure2p 朊病毒结构域的进化变异比 C 末端结构域快得多 (4)。 这可能是由于朊病毒结构域缺乏功能限制，但我们已经证明它对于 Ure2p 的完整功能确实很重要 (5)。 我们建议实际上选择 Ure2p 朊病毒结构域的变异是为了保护细胞免受其他菌株的 URE3 感染。 这类似于 Collinge 的提议，即人类 PrP 中残基 129 (Met/Val) 的多态性在人群中得以维持（或至少曾经），以尽量减少同类相食的影响 (6)。 为了检验这一假设，我们检查了来自酵母属各种杂交品种的 Ure2p。 我们也发现，朊病毒结构域的变异性远远超过 C 端结构域。 此外，对于 URE3 从一个物种的具有 Ure2p 的细胞到表达另一物种的 Ure2p 的细胞的传输，存在很大的“物种障碍”（H. Edskes、L. McCann、R. Wickner，正在准备中）。 有人认为，对于蛋白质的正常功能来说，朊病毒结构域的存在是可有可无的，这意味着朊病毒的形成本身在进化中得到了保留。 然而，我们已经证明，Ure2p 的朊病毒结构域实际上是该蛋白质在氮调节中发挥全部功能所必需的 (7)。 此外，我们发现某些物种的 Ure2p 无法经历朊病毒变化（H. K. Edskes、L. McCann 和 R. Wickner，准备中）。 当然，在这些情况下，我们不能说保留 N 端结构域是为了促进朊病毒形成。 朊病毒“变体”是不同的朊病毒分离株，其中朊病毒蛋白序列相同且宿主相同，但朊病毒产生的表型和/或稳定性或其他可遗传特征不同。 朊病毒变体被认为是由相应淀粉样纤维的结构差异引起的。 我们发现不同的朊病毒变体表现出不同的物种屏障（H. Edskes、L. McCann、R. Wickner，准备中）。 这一结果让人想起，与其他变体相比，TSE 的 BSE 菌株显示出物种屏障的减少，甚至在相同的小鼠中繁殖也是如此。 了解朊病毒变体、物种屏障和淀粉样蛋白结构的关系是这个项目和我们参与的其他项目的一个关键目标。 因此，我们正在制备各种形成朊病毒和非朊病毒的 Ure2 蛋白，并检查它们的淀粉样蛋白形成能力和淀粉样蛋白结构（A. Engel、H. K. Edskes 和 R. Wickner）。 1. Wickner, R. B. URE3 作为一种改变的 URE2 蛋白：酿酒酵母中存在朊病毒类似物的证据。科学 264, 566 - 569 (1994)。 2. R.B. Wickner、H.E. Edskes、F. Shewmaker 和 T. Nakayashiki (2007)。真菌朊病毒：遗传结构和生物学作用。纳特。微生物。修订版 5，611-618。 3. Nakayashiki, T.、Kurtzman, C.P.、Edskes, H.K. 和 Wickner, R.B. (2005)。酵母朊病毒 URE3 和 PSI 是疾病。美国国家科学院院报 102, 10575-10580。 4. Edskes, H.K. 和 Wickner, R.B. (2002)。与全长蛋白质相互作用的酿酒酵母 Ure2p 朊病毒结构域的一部分的保守。过程。国家。阿卡德。科学。美国 99（增补 4），16384-16391。 5. Shewmaker, F.、Mull, L.、Nakayashiki, T.、Maison, D.C. 和 Wickner, R.B. (2007)。在酿酒酵母中，Ure2p 的功能通过其朊病毒结构域得到增强。遗传学 176, 1557 - 1565。 6.科林格，J.（1999）。变异型克雅氏病。柳叶刀 354, 317-323。 7. Shewmaker, F.、Mull, L.、Nakayashiki, T.、Maison, D.C. 和 Wickner, R.B. (2007)。在酿酒酵母中，Ure2p 的功能通过其朊病毒结构域得到增强。遗传学 176, 1557 - 1565。

The word 'prion' means 'infectious protein', a protein which can transmit a disease or trait without the necessity for an accompanying nucleic acid. The prion concept has its origins in studies of the mammalian transmissible spongiform encephalopathies (TSEs), a group of uniformly fatal diseases whose underlying cause appears to be the formation of amyloid composed of the PrP protein. Scrapie of sheep is a TSE that is readily transmitted (by injection) to other sheep, but to goats only after a long incubation period. Subsequent goat to goat transmissions show a much shorter delay. This is the original 'species barrier', and more dramatic versions are well known. Hamster scrapie is not transmissible to mice at all, and mouse scrapie only with difficulty to hamsters. In 1994, we described two prions of the yeast Saccharomyces cerevisiae, called URE3 and PSI, based on a self-propagating inactivation of the Ure2 and Sup35 proteins, respectively (1). Ure2p is a regulator of nitrogen catabolism and URE3 strains show a derepression of the genes normally repressed by this protein. Sup35p is a subunit of the translation termination factor, and PSI+ strains show elevated readthrough of translation termination codons, a phenotype similar to that of a sup35 mutant. In each case, prion formation is the conversion of most of the protein to an amyloid form, a filamentous beta-sheet rich form in which the protein is not available to carry out its normal function (reviewed in ref. 2). We have shown that the URE3 and PSI+ prions are diseases of yeast, detrimental to their survival or propagation such that neither prion is found among 70 wild strains in spite of both prions being infectious (3). In contrast, all the viral or plasmid nucleic acid infectious elements known are found in some fraction of the wild strains (3). We have shown that the variation in evolution of the Ure2p prion domain is much more rapid than the C-terminal domain (4). This could be due to lack of functional constraints on the prion domain, but we have shown that it is indeed important for full function of Ure2p (5). We suggest that variation of the Ure2p prion domain is actually selected in order to protect cells from acquiring a URE3 infection from other strains. This is analogous to the proposal by Collinge that the polymorphism at residue 129 (Met/Val) in the human PrP is (or at least was) maintained in the human population to minimize the effects of cannibalism (6). To test this hypothesis, we have examined the Ure2p's from various cross-breeding species of the genus Saccharomyces. We find that here too, the variability of the prion domain far exceeds that of the C-terminal domain. Moreover, there is a substantial 'species barrier' for transmission of URE3 from cells with Ure2p from one species to cells expressing Ure2p from another species (H. Edskes, L. McCann, R. Wickner, in preparation). It has been suggested that the presence of the prion domains that are dispensable for the usual function of the protein implies that the prion formation itself is being preserved in evolution. However, we have shown that the prion domain of Ure2p is in fact necessary for the full function of the protein in nitrogen regulation (7). Moreover, we find that the Ure2p's of some species are unable to undergo the prion change (H. K. Edskes, L. McCann & R. Wickner, in preparation). Certainly in those cases, one cannot argue that the N-terminal domain is maintained for the purpose of enabling prion formation. Prion 'variants' are different prion isolates in which the prion protein is identical in sequence and the host is identical, but the phenotype produced and/or the stability or other heritable characteristic of the prion is different. Prion variants are believed to result from differences in the structure of the corresponding amyloid fibers. We find that different prion variants show different species barriers (H. Edskes, L. McCann, R. Wickner, in preparation). This result is reminiscent of the reduced species barrier shown by the BSE strain of TSE compared to other variants, even propagated in identical mice. Understanding the relation of prion variants, species barrier and amyloid structure is a key aim of this and other projects in which we are engaged. Thus we are preparing various prion-forming and non-prion forming Ure2 proteins and are examining their amyloid-forming abilities and amyloid structures (A. Engel, H. K. Edskes, and R. Wickner). 1. Wickner, R. B. URE3 as an altered URE2 protein: evidence for a prion analog in S. cerevisiae. Science 264, 566 - 569 (1994). 2. Wickner, R.B., Edskes, H.E., Shewmaker, F., and Nakayashiki, T. (2007). Prions of fungi: inherited structures and biological roles. Nat. Microbiol. Rev. 5, 611-618. 3. Nakayashiki, T., Kurtzman, C.P., Edskes, H.K., and Wickner, R.B. (2005). Yeast prions URE3 and PSI+ are diseases. Proc Natl Acad Sci U S A 102, 10575-10580. 4. Edskes, H.K., and Wickner, R.B. (2002). Conservation of a portion of the S. cerevisiae Ure2p prion domain that interacts with the full - length protein. Proc. Natl. Acad. Sci. U. S. A. 99 (Suppl. 4), 16384-16391. 5. Shewmaker, F., Mull, L., Nakayashiki, T., Masison, D.C., and Wickner, R.B. (2007). Ure2p function is enhanced by its prion domain in Saccharomyces cerevisiae. Genetcs 176, 1557 - 1565. 6. Collinge, J. (1999). Variant Creutzfeldt-Jakob disease. Lancet 354, 317-323. 7. Shewmaker, F., Mull, L., Nakayashiki, T., Masison, D.C., and Wickner, R.B. (2007). Ure2p function is enhanced by its prion domain in Saccharomyces cerevisiae. Genetcs 176, 1557 - 1565.