Chellgren Brian W, Miller Anne-Frances, Creamer Trevor P
Center for Structural Biology, Department of Molecular and Cellular Biochemistry, University of Kentucky, 741 South Limestone Street, Lexington, KL 40536-0509, USA.
J Mol Biol. 2006 Aug 11;361(2):362-71. doi: 10.1016/j.jmb.2006.06.044. Epub 2006 Jul 5.
Nine neurodegenerative diseases, including Huntington's disease, are associated with the aggregation of proteins containing expanded polyglutamine sequences. The end result of polyglutamine aggregation is a beta-sheet-rich deposit. There exists evidence that an important intermediate in the aggregation process involves intramolecular beta-hairpin structures. However, little is known about the starting state, monomeric polyglutamine. Most experimental studies of monomeric polyglutamine have concluded that the backbone is completely disordered. However, such studies are hampered by the inherent tendency for polyglutamine to aggregate. A recent computational study suggested that the glutamine residues in polyglutamine tracts have a significant propensity to adopt the left-handed polyproline II (P(II)) helical conformation. In this work, we use NMR spectroscopy to demonstrate that glutamine residues possess a high propensity to adopt the P(II) conformation. We present circular dichroism spectra that indicate the presence of significant amounts of P(II) helical structure in short glutamine tracts. These data demonstrate that the propensity to adopt the P(II) structure is retained for glutamine repeats of up to at least 15 residues. Although other structures, such as alpha-helices and beta-sheets, become possible at greater lengths, our data indicate that glutamine residues in monomeric polyglutamine have a significant propensity to adopt the P(II) structure, although not necessarily in long contiguous helical stretches. We note that we have no evidence to suggest that the observed P(II) helical structure is a precursor to polyglutamine aggregation. Nonetheless, increased understanding of monomeric polyglutamine structures will aid our understanding of the aggregation process.
包括亨廷顿舞蹈症在内的九种神经退行性疾病与含有扩展型聚谷氨酰胺序列的蛋白质聚集有关。聚谷氨酰胺聚集的最终结果是形成富含β折叠的沉积物。有证据表明，聚集过程中的一个重要中间体涉及分子内β发夹结构。然而，对于起始状态的单体聚谷氨酰胺却知之甚少。大多数关于单体聚谷氨酰胺的实验研究得出结论，其主链是完全无序的。然而，此类研究因聚谷氨酰胺固有的聚集倾向而受到阻碍。最近的一项计算研究表明，聚谷氨酰胺片段中的谷氨酰胺残基具有显著的倾向采用左手聚脯氨酸II（P(II)）螺旋构象。在这项工作中，我们使用核磁共振光谱来证明谷氨酰胺残基具有很高的倾向采用P(II)构象。我们展示的圆二色光谱表明，在短聚谷氨酰胺片段中存在大量的P(II)螺旋结构。这些数据表明，长达至少15个残基的谷氨酰胺重复序列仍保留采用P(II)结构的倾向。尽管在更长的长度下其他结构（如α螺旋和β折叠）也可能形成，但我们的数据表明，单体聚谷氨酰胺中的谷氨酰胺残基具有显著的倾向采用P(II)结构，尽管不一定是长的连续螺旋片段。我们注意到，我们没有证据表明观察到的P(II)螺旋结构是聚谷氨酰胺聚集的前体。尽管如此，对单体聚谷氨酰胺结构的进一步了解将有助于我们理解聚集过程。
