Species Barriers of Yeast Prions

Prion这个词的意思是“感染性蛋白”，一种不需要伴随的核酸就能传播疾病或特征的蛋白质。Prion的概念起源于对哺乳动物可传播海绵状脑病(TSEs)的研究，TSEs是一组一致致命的疾病，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，但只有在长时间潜伏期后才能传播给山羊。随后的山羊间传播显示延迟要短得多。这是最初的“物种屏障”，更戏剧性的版本是众所周知的。仓鼠瘙痒病完全不会传染给老鼠，而老鼠瘙痒病只会很难传染给仓鼠。 1994年，我们描述了酿酒酵母的两个普恩，称为URE3和PSI，分别基于Ure2和Sup35蛋白的自繁殖失活(1)。Ure2p是氮分解代谢的调节器，而URE3菌株表现出通常由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+菌株表现出翻译终止密码子的高通读率，这是一种类似于sup35突变体的表型。在每种情况下，Prion的形成都是将大部分蛋白质转化为淀粉样蛋白，淀粉样蛋白是一种富含丝状β-折叠的形式，在这种形式下蛋白质无法执行其正常功能(参考文献[参考文献])。2)。 我们已经证明URE3和PSI+Pron是酵母的疾病，不利于它们的生存或繁殖，以至于在70个野生菌株中都没有发现Prion，尽管这两个Prion都是有感染性的(3)。相比之下，所有已知的病毒或质粒核酸感染因子都在部分野生株中发现(3)。我们已经表明，Ure2p蛋白结构域的进化比C-末端结构域的变化要快得多(4)。这可能是由于Pron结构域缺乏功能限制，但我们已经证明，这对于Ure2p(5)的完整功能确实很重要。 我们认为，Ure2p蛋白结构域的变异实际上是为了保护细胞免受来自其他菌株的URE3感染。这类似于Colinger的建议，即人类PrP第129位(Met/Val)的多态在人类群体中保持(或至少保持)，以最大限度地减少同类相食的影响(6)。为了验证这一假设，我们研究了来自酵母属不同杂交物种的Ure2p。我们还发现，在这里，Prion结构域的可变性远远超过C-末端结构域。此外，对于URE3从一个物种的带有Ure2p的细胞传播到另一个物种的表达Ure2p的细胞(H.Edskes，L.McCann，R.Wickner，正在准备中)来说，存在着实质性的“物种屏障”。 有人认为，对蛋白质的正常功能来说是可有可无的Pron结构域的存在意味着Pron的形成本身在进化过程中被保存下来。然而，我们已经证明，Ure2p的Prion结构域实际上是蛋白质在氮调节中充分发挥功能所必需的(7)。此外，我们发现一些物种的Ure2p不能经历蛋白的变化(H.K.Edskes，L.McCann和R.Wickner，正在准备中)。当然，在这些情况下，人们不能争辩说，N-末端结构域的维持是为了使Pron形成。 Prion变异体是不同的Prion分离株，其中Prion蛋白在序列上是相同的，宿主是相同的，但产生的表型和/或Prion的稳定性或其他可遗传特性是不同的。Prion变种被认为是由于相应的淀粉样纤维结构的差异造成的。我们发现不同的Prion变体表现出不同的物种障碍(H.Edskes，L.McCann，R.Wickner，正在准备中)。这一结果使人想起，与其他变异株相比，即使在相同的小鼠中繁殖，疯牛病菌株的物种屏障也有所减少。了解普恩变异体、物种屏障和淀粉样蛋白结构之间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Prion形成和非Prion形成的Ure2蛋白，并检测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：在酿酒酵母中发现了普恩类似物的证据。《科学》264,566-569(1994)。 2.Wickner，R.B.，Edskes，H.C.，Shewaker，F.和Nakayashiki，T.(2007)。真菌的普利子：遗传结构和生物学作用。纳特。微生物。版本5,611-618。 3.Nakayashiki，T.，Kurtzman，C.P.，Edskes，H.K.和Wickner，R.B.(2005)。酵母蛋白URE3和PSI+是一种疾病。《南洋理工大学学报》S A102,10575-10580。 4.Edskes，H.K.和Wickner，R.B.(2002)。与全长蛋白相互作用的酿酒酵母Ure2p蛋白结构域的一部分的保守。程序娜塔莉。阿卡德。SCI。美国第99条(补充)4)，16384-16391。 5.Shewaker，F.，Mull，L.，Nakayashiki，T.，Masison，D.C.和Wickner，R.B.(2007年)。在酿酒酵母中，Ure2p的功能被其Pron结构域所增强。Genetcs 176,1557-1565。 6.Colinger，J.(1999)。变异型克雅病。《柳叶刀》354,317-323。 7.Shewaker，F.，Mull，L.，Nakayashiki，T.，Masison，D.C.和Wickner，R.B.(2007年)。在酿酒酵母中，Ure2p的功能被其Pron结构域所增强。Genetcs 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.