Rowling Pamela J E, Sivertsson Elin M, Perez-Riba Albert, Main Ewan R G, Itzhaki Laura S
Department of Pharmacology, University of Cambridge, Tennis Court Road, Cambridge CB2 1PD, U.K.
School of Biological and Chemical Sciences, Queen Mary, University of London, London E1 4NS, U.K.
Biochem Soc Trans. 2015 Oct;43(5):881-8. doi: 10.1042/BST20150099.
Studying protein folding and protein design in globular proteins presents significant challenges because of the two related features, topological complexity and co-operativity. In contrast, tandem-repeat proteins have regular and modular structures composed of linearly arrayed motifs. This means that the biophysics of even giant repeat proteins is highly amenable to dissection and to rational design. Here we discuss what has been learnt about the folding mechanisms of tandem-repeat proteins. The defining features that have emerged are: (i) accessibility of multiple distinct routes between denatured and native states, both at equilibrium and under kinetic conditions; (ii) different routes are favoured for folding compared with unfolding; (iii) unfolding energy barriers are broad, reflecting stepwise unravelling of an array repeat by repeat; (iv) highly co-operative unfolding at equilibrium and the potential for exceptionally high thermodynamic stabilities by introducing consensus residues; (v) under force, helical-repeat structures are very weak with non-co-operative unfolding leading to elasticity and buffering effects. This level of understanding should enable us to create repeat proteins with made-to-measure folding mechanisms, in which one can dial into the sequence the order of repeat folding, number of pathways taken, step size (co-operativity) and fine-structure of the kinetic energy barriers.
由于球状蛋白具有拓扑复杂性和协同性这两个相关特征，研究其蛋白质折叠和蛋白质设计面临重大挑战。相比之下，串联重复蛋白具有由线性排列基序组成的规则且模块化的结构。这意味着，即使是巨大的重复蛋白，其生物物理学特性也非常适合进行剖析和合理设计。在此，我们讨论关于串联重复蛋白折叠机制已了解到的情况。已出现的决定性特征如下：(i) 在平衡态和动力学条件下，变性态与天然态之间存在多条不同的途径；(ii) 与去折叠相比，折叠更倾向于不同的途径；(iii) 去折叠能垒较宽，反映出重复序列逐个逐步解开；(iv) 在平衡态下高度协同去折叠，通过引入共有残基有潜力实现极高的热力学稳定性；(v) 在受力情况下，螺旋重复结构非常脆弱，非协同去折叠导致弹性和缓冲效应。这种理解水平应使我们能够创建具有定制折叠机制的重复蛋白，其中可以在序列中设定重复折叠的顺序、所采用途径的数量、步长（协同性）以及动能垒的精细结构。
