Devlin Glyn L, Knowles Tuomas P J, Squires Adam, McCammon Margaret G, Gras Sally L, Nilsson Melanie R, Robinson Carol V, Dobson Christopher M, MacPhee Cait E
Cavendish Laboratory, University of Cambridge, JJ Thomson Avenue, Cambridge CB3 0HE, UK.
J Mol Biol. 2006 Jul 7;360(2):497-509. doi: 10.1016/j.jmb.2006.05.007. Epub 2006 May 17.
Amyloid fibrils are typically rigid, unbranched structures with diameters of approximately 10 nm and lengths up to several micrometres, and are associated with more than 20 diseases including Alzheimer's disease and type II diabetes. Insulin is a small, predominantly alpha-helical protein consisting of 51 residues in two disulfide-linked polypeptide chains that readily assembles into amyloid fibrils under conditions of low pH and elevated temperature. We demonstrate here that both the A-chain and the B-chain of insulin are capable of forming amyloid fibrils in isolation under similar conditions, with fibrillar morphologies that differ from those composed of intact insulin. Both the A-chain and B-chain fibrils were found to be able to cross-seed the fibrillization of the parent protein, although these reactions were substantially less efficient than self-seeding with fibrils composed of full-length insulin. In both cases, the cross-seeded fibrils were morphologically distinct from the seeding material, but shared common characteristics with typical insulin fibrils, including a very similar helical repeat. The broader distribution of heights of the cross-seeded fibrils compared to typical insulin fibrils, however, indicates that their underlying protofilament hierarchy may be subtly different. In addition, and remarkably in view of this seeding behavior, the soluble forms of the A-chain and B-chain peptides were found to be capable of inhibiting insulin fibril formation. Studies using mass spectrometry suggest that this behavior might be attributable to complex formation between insulin and the A-chain and B-chain peptides. The finding that the same chemical form of a polypeptide chain in different physical states can either stimulate or inhibit the conversion of a protein into amyloid fibrils sheds new light on the mechanisms underlying fibril formation, fibril strain propagation and amyloid disease initiation and progression.
淀粉样纤维通常是刚性的、无分支的结构，直径约为10纳米，长度可达几微米，与包括阿尔茨海默病和II型糖尿病在内的20多种疾病有关。胰岛素是一种小的、主要为α螺旋的蛋白质，由51个残基组成，存在于两条通过二硫键连接的多肽链中，在低pH值和高温条件下很容易组装成淀粉样纤维。我们在此证明，胰岛素的A链和B链在相似条件下都能够单独形成淀粉样纤维，其纤维形态与由完整胰岛素组成的纤维不同。发现A链和B链纤维都能够交叉引发亲本蛋白的纤维化，尽管这些反应的效率远低于用全长胰岛素组成的纤维进行的自引发。在这两种情况下，交叉引发的纤维在形态上与引发材料不同，但与典型的胰岛素纤维具有共同特征，包括非常相似的螺旋重复。然而，与典型的胰岛素纤维相比，交叉引发的纤维高度分布更宽，这表明它们潜在的原纤维层次结构可能略有不同。此外，鉴于这种引发行为，值得注意的是，发现A链和B链肽的可溶形式能够抑制胰岛素纤维的形成。使用质谱的研究表明，这种行为可能归因于胰岛素与A链和B链肽之间形成复合物。同一多肽链在不同物理状态下的相同化学形式既能刺激也能抑制蛋白质转化为淀粉样纤维，这一发现为纤维形成、纤维菌株传播以及淀粉样疾病的起始和进展的潜在机制提供了新的线索。
