Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK.
J Mol Biol. 2012 Feb 10;416(1):137-47. doi: 10.1016/j.jmb.2011.12.012. Epub 2011 Dec 13.
The study of the folding of single domains, in the context of their multidomain environment, is important because more than 70% of eukaryotic proteins are composed of multiple domains. The structures of the tandem immunoglobulin (Ig) domain pairs A164-A165 and A168-A169, from the A-band of the giant muscle protein titin, reveal that they form tightly associated domain arrangements, connected by a continuous β-strand. We investigate the thermodynamic and kinetic properties of these tandem domain pairs. While A164-A165 apparently behaves as a single cooperative unit at equilibrium, unfolding without the accumulation of a large population of intermediates, domains in A168-A169 behave independently. Although A169 appears to be stabilized in the tandem protein, we show that this is due to nonspecific stabilization by extension. We elucidate the folding and unfolding pathways of both tandem pairs and show that cooperativity in A164-A165 is a manifestation of the relative refolding and unfolding rate constants of each individual domain. We infer that the differences between the two tandem pairs result from a different pattern of interactions at the domain/domain interface.
在多结构域环境中研究单结构域的折叠情况非常重要，因为超过 70%的真核生物蛋白由多个结构域组成。来自巨大肌肉蛋白titin A 带的串联免疫球蛋白（Ig）结构域对 A164-A165 和 A168-A169 的结构表明，它们形成紧密相关的结构域排列，由连续的β-链连接。我们研究了这些串联结构域对的热力学和动力学特性。虽然 A164-A165 在平衡时显然表现为单个协同单元，在没有大量中间产物积累的情况下发生解折叠，但 A168-A169 中的结构域独立行为。尽管 A169 在串联蛋白中似乎得到了稳定，但我们表明这是由于延伸引起的非特异性稳定。我们阐明了这两个串联结构域对的折叠和展开途径，并表明 A164-A165 中的协同作用是每个单独结构域的相对折叠和展开速率常数的表现。我们推断这两个串联结构域对之间的差异是由于结构域/结构域界面处的相互作用模式不同。
