Super-Long, Single Alpha Helices: A Mechanical Unfolding Study

Abstract

Most α-helices in proteins are short and found in positions where stabilising interactions exist with neighbouring secondary structure elements. However, a number of very long, isolated, single α-helical (SAH) domains have been discovered. For example, >100-residue unbroken helices have been observed in caldesmon and certain myosins. This novel structural motif contains a high proportion of charged E, R and K residues, which appear as alternating patches of like-charged residues throughout the sequence. The resulting salt bridge interactions between E and R/K sidechains are thought to stabilise and stiffen the straight helical structure, allowing the SAH to act as a spacer between two flanking functional domains. Here we use single molecule force spectroscopy and molecular simulation to investigate the mechanical unfolding behaviour of the ∼100 residue SAH domain from myosin 10. Both methods indicate a globally non-cooperative unfolding process, with unfolding occurring below ∼50 pN. Simulations suggest that the SAH domain does differ from a non-charged helix, not only in the stability of the helix but also in the unfolding characteristics under application of force. Enhanced local bonding interactions in SAH domains increases their resilience to force above the baseline level set by a non-charged helix.