Yardstick Measures Cysteines On The Move

Relying on tiny tugs from an atomic force microscope, a new technique shows promise for measuring the dynamics of cysteine residues’ reactivity in proteins (Nat. Chem., DOI: 10.1038/nchem.1155). Many proteins derive structural integrity from multiple disulfide bonds between cysteine residues. But in nascent or actively folding proteins, cysteines are on the move, pairing transiently before switching partners in the search for a stable configuration. Julio M. Fernández of Columbia University and colleagues sought to disentangle the dynamics of an individual cysteine in this type of complex environment. They tethered a section of the muscle protein titin containing multiple disulfides to a custom atomic force microscopy (AFM) setup and yanked on it in the presence of a nucleophile to uncage a reactive cysteine thiol group. They could tell which of the two sulfur atoms in a nearby disulfide was attacked by the cysteine through AFM measurements, which revealed how far the resulting protein product could extend. The team says its technique could also be used to monitor competing pathways in other reactions that involve thiol groups.