Decoupling of size and shape fluctuations in heteropolymeric sequences reconciles discrepancies in SAXS vs. FRET measurements

Abstract

Significance Conformational properties of unfolded and intrinsically disordered proteins (IDPs) under native conditions are important for understanding the details of protein folding and the functions of IDPs. The average dimensions of these systems are quantified using the mean radius of gyration and mean end-to-end distance, measured by small-angle X-ray scattering (SAXS) and single-molecule Förster resonance energy transfer (smFRET), respectively, although systematic discrepancies emerge from these measurements. Through holistic sets of studies, we find that the disagreements arise from chemical heterogeneity that is inherent to heteropolymeric systems. This engenders a decoupling between different measures of overall sizes and shapes, thus leading to discrepant inferences based on SAXS vs. smFRET. Our findings point the way forward to obtaining comprehensive descriptions of ensembles of heterogeneous systems.