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

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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.