In the course of this discovery, the researchers also observed something unexpected: as long as they stopped stretching the molecules before they were completely unfolded, the extension was fully and immediately reversible.
The ankyrin repeats showed no sign of wear after multiple stretch-relaxation cycles over a large range of applied forces, the team found. When extended past their breaking points, the molecules exhibited a saw-tooth pattern of regularly spaced force peaks as individual repeats unraveled one by one, they said.
After this complete unfolding of the protein, the researchers observed that ankyrin repeats were able to rapidly refold and that process of refolding generated significant force, they reported.
"This, to our knowledge, is the first report of the magnitude of the refolding force of a protein domain obtained through direct measurements on single molecules," the researchers said.
The team speculates that the linear elasticity of 24 ankyrin repeats may play an important biological role in adjusting ankyrin-associated transporters in response to mechanical strain or in generating tension in cell membranes. The unusually strong tendency of ankyrin repeats to refold might also play a significant role in the function of proteins having fewer repeats.
"Ankyrins appear to have a very unusual structural design ?consisting of short anti-parallel alpha helices that self-assemble into stacks -- that allows them to quickly and robustly unfold and refold," Bennett said. "It's hard to believe, but it happens. You have to wonder whether their structure plays an important functional role in sensing forces." Alpha helices are common prot