The ability of proteins to sense membrane curvature is essential to cellular function. All known sensing mechanisms rely on protein domains with specific structural features such as wedge-like amphipathic helices and crescent-shaped BAR domains. Yet many proteins that contain these domains also contain large intrinsically disordered regions. Here we report that disordered domains are themselves potent sensors of membrane curvature. Comparison of Monte Carlo simulations with in vitro and live-cell measurements demonstrates that the polymer-like behavior of disordered domains found in endocytic proteins drives them to partition preferentially to convex membrane surfaces, which place fewer geometric constraints on their conformational entropy. Further, proteins containing both structured curvature sensors and disordered regions are more than twice as curvature sensitive as their respective structured domains alone. These findings demonstrate an entropic mechanism of curvature sensing that is independent of protein structure and illustrate how structured and disordered domains can synergistically enhance curvature sensitivity.

Receptor internalization by endocytosis regulates diverse cellular processes, from the rate of nutrient uptake to the timescale of essential signaling events. The established view is that internalization is tightly controlled by specific protein-binding interactions. However, recent work suggests that physical aspects of receptors influence the process in ways that cannot be explained by biochemistry alone. Specifically, work from several groups suggests that increasing the steric bulk of receptors may inhibit their uptake by multiple types of trafficking vesicles. How do biochemical and biophysical factors work together to control internalization? Here, we show that receptor uptake is well described by a thermodynamic trade-off between receptor-vesicle binding energy and the entropic cost of confining receptors within endocytic vesicles. Specifically, using large ligands to acutely increase the size of engineered variants of the transferrin receptor, we demonstrate that an increase in the steric bulk of a receptor dramatically decreases its probability of uptake by clathrin-coated structures. Further, in agreement with a simple thermodynamic analysis, all data collapse onto a single trend relating fractional occupancy of the endocytic structure to fractional occupancy of the surrounding plasma membrane, independent of receptor size. This fundamental scaling law provides a simple tool for predicting the impact of receptor expression level, steric bulk, and the size of endocytic structures on receptor uptake. More broadly, this work suggests that bulky ligands could be used to drive the accumulation of specific receptors at the plasma membrane surface, providing a biophysical tool for targeted modulation of signaling and metabolism from outside the cell.

Receptor–receptor competition for uptake reduces the probability of receptor partitioning into endocytic structures as described by an equilibrium thermodynamics model.

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melittin, appears to favor lipidation, where as for drugs, the depth of bilayer partitioning appears to be a crucial factor. In membranes with saturated lipids such as DPPC or DMPC, melittin undergoes little or no lipidation, whereas with unsaturated lipids such as POPC, modest lipidation is found. Cholesterol incorporation into all membrane types leads to striking increases in lipidation activity, alongside changes in the chemical selectivity of the transfer process. For some POPC/cholesterol mixtures, less than 50% of the original unmodified melittin is present after 24 hours. The level of activity does not correlate with the affinity of melittin for each membrane type. Rather the curvature modulus of the membrane is a better predictor of membrane activity. We hypothesise that the qualitative relationship between curvature modulus and lipidation activity is a consequence of changes in the penetration depth of melittin. Access of water to lipidation intermediates may also influence lipidation rates. 108-PlatIntrinsically Disordered Proteins Sense Membrane Curvature From endocytic vesicles to cytoskeletal protrusions, the ability of proteins to sense membrane curvature is essential for efficient initiation and assembly of curved membrane structures. To date, all known mechanisms rely on protein domains with specific structural features such as wedge-like amphipathic helices and crescent-shaped BAR domains. Many proteins that contain these structured domains also contain large intrinsically disordered regions. Nonetheless studies of curvature sensing have focused on structured domains in isolation, based on the assumption that curvature sensing requires structural order. In contrast, here we report that disordered domains are themselves potent sensors of membrane curvature. Comparison of Monte Carlo simulations with quantitative in vitro and live-cell measurements demonstrates that the polymer-like behavior of disordered domains found in endocytic proteins, including AP180, Epsin1, and Amphiphysin1, drives them to partition preferentially to convex membrane surfaces, which place fewer geometric constraints on their conformational entropy. For highly charged disordered proteins, electrostatic effects stiffen the peptide chain, reducing entropic curvature sensing. However, in these cases, increased repulsion between the disordered protein and the membrane surface give rise to an alternative, electrostatically derived mechanism of curvature sensing. Finally, full-length endocytic proteins, which contain both structured curvature sensors and disordered regions, are more than twice as curvature sensitive as their respective structured domains alone. These findings demonstrate curvature sensing mechanisms that are independent of protein structure and illustrate how structured and disordered domains can collaborate to synergistically enhance curvature sensitivity. MutSg plays a role in meiotic recombination facilitating crossover formation between homologous chromosomes. Failure to form crossovers leads to improper segre...