Low-Force Elasticity Reveals Complex Structure of an Intrinsically Disordered Protein*

Intrinsically disordered proteins (IDPs) do not possess a well-defined three-dimensional structure, which presents a challenge to investigating their conformations. To overcome this challenge, we explore low-force polymeric elasticity as a means to quantify structural properties. Here, using a high-resolution single-molecule magnetic tweezer, we stretch a polypeptide construct derived from the neurofilament tail domains and study its conformations. At low forces, the construct behaves as an ideal coil instead of a self-avoiding coil. This is surprising given the IDP’s high net charge, suggesting the presence of weak long-range attractive interactions. In addition, the measured persistence length is longer than expected for a random polypeptide chain. This result suggests the IDP has residual structure, which stiffen the chain at low force. We further probe the response of long-range attraction and chain stiffening to changes in salts and denaturants. Our data reveals a rich elastic behavior and complex structure in a nominally disordered chain.