(505e) Intrinsically Disordered Proteins As Sensors of Membrane Curvature

The ability of proteins to detect membrane curvature is essential for the initiation and growth of endocytic structures. To date, all known mechanisms of curvature sensing rely on specific protein structural features. Key examples include wedge-like amphipathic helices, which insert into defects on the surfaces of highly curved membranes and crescent-shaped BAR domains, which bind with greatest affinity to highly curved membranes that match their inherent curvature. In contrast, here we report a novel mechanism of membrane curvature sensing that arises from a lack of protein structure. Specifically, our results demonstrate that intrinsically disordered protein (IDP) domains, which lack a well-defined secondary structure, are highly sensitive to membrane curvature. In particular, quantitative measurements of protein-membrane binding as a function of membrane curvature reveal that the chain entropy of IDPs drives them to bind more strongly to highly curved membrane surfaces, which pose fewer steric restrictions in comparison to flat membranes. Using a tethered vesicle assay to quantify protein binding as a function of membrane curvature, the IDP C-terminal domains of the endocytic adaptor proteins AP180 and Epsin1 bound to highly curved lipid vesicles (20-50 nm in diameter) with approximately 5-fold greater affinity than to vesicles of lower curvature (100-200 nm in diameter). This level of curvature sensitivity is comparable to that observed using the same assay to examine the amphipathic helix-containing ENTH domain and the BAR domain of Amphiphysin. These findings are particularly important since IDP domains are found within many of the best studied endocytic proteins, including the adaptors of the clathrin pathway, the COPII pathway and others. In particular, this work suggests that entropic mechanisms of membrane curvature sensing - which arise from changes in protein conformational entropy rather than protein structure - may provide a highly potent, yet previously unknown mechanism of membrane curvature sensing. These findings elucidate the interplay between structured and unstructured domains in curvature sensing proteins, and paradoxically illustrate how a lack of protein structure can result in protein functionality.