Science 342, 741–743 (2013)

Credit: © 2013 AAAS

Titin — the largest known protein — is a chain of nearly 300 (immunoglobulin-like) folded protein domains that acts as a molecular spring in the contraction of striated muscle. Titin's function thus depends on how it mechanically unfolds, which has been studied by both force microscopy and spectroscopy (by pulling the protein after attaching one of its ends to a cantilever), as well as by steered molecular dynamics simulations. However, because atomistic simulations of large proteins can only reach a few microseconds with today's computational capabilities, the pulling speed needed to see domain unfolding is orders of magnitude larger than the fastest protein-pulling experiments. Now, Felix Rico and colleagues are able to compare force-spectroscopy and simulation data by using short cantilevers with small viscous damping, which makes them about three orders of magnitude faster than conventional ones. The researchers find that at pulling forces up to about 100 pN the unfolding and refolding of a β-strand pair of an immunoglobulin domain occurs at least at a rate of 105 s−1, much faster than earlier predictions.