Probing the molecular origin of native-state flexibility in repeat proteins

In contrast to globular proteins, the structure of repeat proteins is dominated by a regular set of short-range interactions. This property may confer on the native state of such proteins significant elasticity. We probe the molecular origin of the spring-like behavior of repeat proteins using a designed tetratricopeptide repeat protein with three repeat units (CTPR3). Single-molecule fluorescence studies of variants of the protein with FRET pairs at different positions show a continuous expansion of the folded state of CTPR3 at low concentrations of a chemical denaturant, preceding the all-or-none transition to the unfolded state. This remarkable native-state expansion can be explained quantitatively by a reduction in the spring constant of the structure. Circular dichroism and tryptophan fluorescence spectroscopy further show that the expansion does not involve either unwinding of CTPR3 helices or unraveling of interactions within repeats. These findings point to hydrophobic inter-repeat contacts as the source of the elasticity of repeat proteins.