A molecular understanding of how stable single alpha helical domains behave as constant force springs in proteins.

细胞中的蛋白质通常会受到力的作用。我们最近发现了一种特殊的蛋白质结构域，它有助于分离单个蛋白质中的不同结构域，我们现在认为，当它们暴露在细胞内的力中时，它也可以保护它们不会展开。这个新结构域是一种特殊类型的α螺旋，是蛋白质折叠的两种主要类型之一。与其他阿尔法螺旋不同，这种类型的螺旋本身高度稳定，其弯曲刚度也相当高，这意味着它可以充当“间隔物”；当它被夹在蛋白质的两个结构域之间时，就像举重杠铃中两个球之间相当灵活的杆子。它通过定义的量来分隔这些结构域，这仅仅取决于它的长度，因为每个残基在一个阿尔法螺旋中被分开0.15 nm，因此，残基的数量越多，间隔就越长。我们称这种类型的螺旋为‘SAH’结构域(表示稳定的单个α螺旋)。我们在这个SAH领域的最新数据表明，虽然它的弯曲刚度相当高，但如果它沿着长度被拉(受力)，那么它就会逐渐解体，并且在这样做的过程中，它将力保持在恒定的水平。这与弹簧有很大的不同，在弹簧中，随着弹簧末端的拉开，力会继续上升。如果SAH结构域起到弹簧的作用，那么如果作用力上升到足够高的水平，相邻的蛋白质结构域也会有展开的危险，从而破坏它们的功能。相反，通过保持恒定的力，当SAH展开和延长时，相邻的蛋白质结构域被保护，当它们受力时，不会展开，也不会从它们相互作用的蛋白质复合体中分离出来。我们现在计划做的是确定SAH领域到底在什么水平的力量下展开，如果它暴露在一个细胞中的这种力量水平下，如果是的话，如果这些力量真的像我们怀疑的那样展开SAH领域并使其展开。我们还想确定SAH结构域的结构，了解典型SAH结构域序列中带电氨基酸是如何相互作用的，这样我们就可以在分子水平上了解氨基酸是如何相互作用的，并使这种特殊类型的α螺旋如此稳定。这项新工作将使我们对SAH结构域的结构和功能有前所未有的了解，使我们能够在未来利用其在人工蛋白质中的特性。

Proteins in cells are commonly exposed to forces. We have recently discovered a specialised protein domain that helps to separate different domains in a single protein, and we now think that it also protects them from unfolding when they are exposed to forces within the cell. This novel domain is a special type of alpha helix, one of the two main types of protein fold. Unlike other alpha helices, this type is highly stable by itself and its bending stiffness is reasonably high, which means it can act as a 'spacer'; when sandwiched between two domains in a protein, rather like a fairly flexible rod between two balls, in a weightlifting barbell. It separates those domains by a defined amount that simply depends on its length, as each residue is separated by 0.15nm in an alpha helix, and thus, the higher the number of residues, the longer spacer. We call this type of helix a 'SAH' domain (for stable single alpha helix). Our latest data on this SAH domain suggest that while its bending stiffness is reasonably high, if it is pulled on (experiences forces) along its length, then it progressively unravels, and as it does so, it maintains force at a constant level. This is rather different to a spring, where the force would continue to rise as the ends of the spring are pulled apart. If the SAH domain acted as a spring, there would be a danger that the adjacent protein domains would also unfold if the force rose to a high enough level, damaging their function. In contrast, by maintaining a constant force, while the SAH is unfolding and elongating, the adjacent protein domains are protected from unfolding, and from detaching from the protein complex that they interact with, when they are exposed to forces. What we now plan to do is to determine exactly what level of forces the SAH domain unfolds under, if it is exposed to this level of forces in a cell, and if so, if those forces really do unfold the SAH domain and make it unfold, as we suspect. We also want to determine the structure of the SAH domain, to understand how the charged amino acids in a typical SAH domain sequence interact, so we can understand at a molecular level, how the amino acids interact and enable this special type of alpha helix to be so stable. This new work will give us unprecedented insight into the structure and function of the SAH domain, enabling us to exploit its properties in artificial proteins in the future.