Clustering on Membranes: Fluctuations and More

Membrane remodelling 1-5 plays an important role in cellular tasks such as endocytosis, vesiculation and protein sorting, and in the biogenesis of organelles such as the endoplasmic reticulum or the Golgi apparatus. It is well established that the remodelling process is aided by specialized proteins that can sense 4 as well as create 6 membrane curvature, and trigger tubulation 7-9 when added to synthetic liposomes. Because the energy needed for such largescale changes in membrane geometry significantly exceeds the binding energy between individual proteins and between protein and membrane, cooperative action is essential. It has recently been suggested 10,11 that curvature-mediated attractive interactions could aid cooperation and complement the effects of specific binding events on membrane remodelling. But it is difficult to experimentally isolate curvature-mediated interactions from direct attractions between proteins. Moreover, approximate theories predict repulsion between isotropically curving proteins 12-15 . Here we use coarse-grained membrane simulations to show that curvature-inducing model proteins adsorbed on lipid bilayer membranes can experience attractive interactions that arise purely as a result of membrane curvature. We find that once a minimal local bending is realized, the effect robustly drives protein cluster formation and subsequent transformation into vesicles with radii that correlate with the local curvature imprint. Owing to its universal nature, curvature-mediated attraction can operate even between proteins lacking any specific interactions, such as newly synthesized and still immature membrane proteins in the endoplasmic reticulum.

Membrane proteins can deform the lipid bilayer in which they are embedded. If the bilayer is treated as an elastic medium, then these deformations will generate elastic interactions between the proteins. The interaction between a single pair is repulsive. However, for three or more proteins, we show that there are nonpairwise forces whose magnitude is similar to the pairwise forces. When there are five or more proteins, we show that the nonpairwise forces permit the existence of stable protein aggregates, despite their pairwise repulsions.

Nature communications, 2015

Assembly of highly curved membrane structures is essential to cellular physiology. The prevailing view has been that proteins with curvature-promoting structural motifs, such as wedge-like amphipathic helices and crescent-shaped BAR domains, are required for bending membranes. Here we report that intrinsically disordered domains of the endocytic adaptor proteins, Epsin1 and AP180 are highly potent drivers of membrane curvature. This result is unexpected since intrinsically disordered domains lack a well-defined three-dimensional structure. However, in vitro measurements of membrane curvature and protein diffusivity demonstrate that the large hydrodynamic radii of these domains generate steric pressure that drives membrane bending. When disordered adaptor domains are expressed as transmembrane cargo in mammalian cells, they are excluded from clathrin-coated pits. We propose that a balance of steric pressure on the two surfaces of the membrane drives this exclusion. These results prov...

Scientific reports, 2016

The interplay of membrane proteins is vital for many biological processes, such as cellular transport, cell division, and signal transduction between nerve cells. Theoretical considerations have led to the idea that the membrane itself mediates protein self-organization in these processes through minimization of membrane curvature energy. Here, we present a combined experimental and numerical study in which we quantify these interactions directly for the first time. In our experimental model system we control the deformation of a lipid membrane by adhering colloidal particles. Using confocal microscopy, we establish that these membrane deformations cause an attractive interaction force leading to reversible binding. The attraction extends over 2.5 times the particle diameter and has a strength of three times the thermal energy (-3.3 kBT). Coarse-grained Monte-Carlo simulations of the system are in excellent agreement with the experimental results and prove that the measured interact...

Biophysical Journal, 2010

The complex role of plasma membrane structure in orchestrating receptormediated signal transduction is addressed in collaborative studies investigating how antigen crosslinking of IgE-receptors on mast cells initiates signaling pathways leading to multiple cellular responses. Segregation of liquid ordered regions from disordered regions of the plasma membrane provides protection from transmembrane phosphatases and thereby a mechanism for crosslinking-dependent phosphorylation of IgE-receptors by active Lyn kinase in the first signaling event. Defined clustering of IgE-receptors with patterned lipid bilayers enables fluorescence visualization of co-redistributing signaling components with spatial resolution on the micron scale. Nanoscale resolution of clustering components is visualized with scanning electron microscopy and super-resolution fluorescence microscopy. Single molecule dynamics can be characterized in nanofabricated devices. These integrated approaches for examining membrane structural heterogeneity and functional consequences will be discussed.

Essays in Biochemistry, 2015

The understanding of lipid membranes and their organization has undergone significant development with better techniques and therefore more resolved experiments. Many new factors and organizing principles have been discovered, and interplay between these factors is expected to result in rich functional behaviours. The major factors regulating the lateral membrane heterogeneity, apart from the well-studied phase separation, are cytoskeleton pinning, clustering of lipids and curvature. These factors are effective means to create membrane domains that provide rich biological functionality. We review the recent advances and concepts of membrane heterogeneity organization by curvature, cytoskeleton and clustering proteins.

Biophysical Journal, 2011

The polymorphism of eukaryotic cellular membranes is a tightly regulated and well-conserved phenotype. Recent data have revealed important regulatory roles of membrane curvature on the spatio-temporal localization of proteins and in membrane fusion. Here we quantified the influence of membrane curvature on the efficiency of intermembrane docking reactions. Using fluorescence microscopy, we monitored the docking of single vesicle-vesicle pairs of different diameter (30-200 nm) and therefore curvature, as mediated by neuronal soluble N-ethylmaleimide-sensitive factor attachment protein receptors (SNAREs) and streptavidin-biotin. Surprisingly, the intermembrane docking efficiency exhibited an~30-60 fold enhancement as a function of curvature. In comparison, synaptotagmin and calcium accelerate SNARE-mediated fusion in vitro by a factor of 2-10. To explain this finding, we formulated a biophysical model. On the basis of our findings, we propose that membrane curvature can regulate intermembrane tethering reactions and consequently any downstream process, including the fusion of vesicles and possibly viruses with their target membranes.

Physical Review Letters, 2004

We analyze the height undulations of a membrane due to fluctuations in the force generated by membrane-bound proteins that induce normal motion or bending. We compare our results to the results of experiments on red blood cells and vesicles with incorporated active proton pumps. We treat these proteins as having an intrinsic time scale for the force generation or conformational change, leading to nonthermal membrane fluctuations. We find that the active fluctuations are inversely proportional to the viscosity of the surrounding fluid. This highlights some universal features of active membrane undulations.

2014