Session 2: Unstructured proteins in health and disorders L2.2 Single-molecule spectroscopy of intrinsically disordered proteins

In contrast to folded proteins, intrinsically disordered proteins (IDPs) lack a stable three-dimensional structure but, nevertheless, are involved in many biological processes and have a crucial role in various human diseases, including degenerative neuropathies, amyloidosis, and cancer. I will show how advanced single-molecule spectroscopy in combination with Förster resonance energy transfer (FRET) can be used to investigate the large structural heterogeneity and pronounced dynamics of IDPs by mapping intramolecular distance distributions and quantifying the global relaxation dynamics of the chain in the sub-microsecond range. In combination with concepts of polymer physics, single molecule FRET experiments have allowed us, e.g., to explain the effect of electrostatics on the expansion of IDPs and to identify the contribution of internal friction to unfolded-state dynamics. Here I will focus on the use of single-molecule FRET spectroscopy to probe the contribution of charge distribution and metal ion interactions on three different highly acidic IDPs: starmaker, starmaker-like, and prothymosin alpha. Interestingly, for all cases, we observe that divalent ions induce more collapsed conformations than monovalent ions at the same ionic strength, indicating preferential interactions of the sequences with calcium and magnesium compared to sodium and potassium and suggesting a common principle underlying the interaction of counterions with charged residues. The experimental observations can be rationalized in terms of a polyelectrolyte model that takes the competing adsorption of monovalent and divalent ions into account explicitly and allows the fractions of free and bound ions to be estimated. The preferential condensation of divalent ions on negatively charged residues may be an important first step in the assembly of biominerals. Lectures