@article {134601, title = {Clustering of lipids driven by integrin.}, journal = {Soft Matter}, volume = {19}, year = {2023}, month = {2023 Sep 13}, pages = {6814-6824}, abstract = {

Integrin is an important transmembrane receptor protein which remodels the actin network and anchors the cell membrane towards the extracellular matrix mechanochemical pathways. The clustering of specific lipids and lipid-anchored proteins, which is essential for a certain type of endocytosis process, is facilitated at integrin-mediated active regions. To study this, we propose a minimal exactly solvable model which includes the interplay of stochastic shuttling between integrin on and off states with the intrinsic dynamics of the membrane. We propose a two-step mechanism in which the integrin induces an aster-like arrangement in the actin network, followed by clustering of lipids in that region. We obtain an analytic expression for the deformation and local membrane velocity, and thereby the evolution of clustering mediated by a single integrin. The deformation evolves nonmonotonically and its dependence on the stochastic shuttling timescales and membrane properties is elucidated. Our estimates of the area of the deformed region and the number of lipids in it indicate strong clustering.

}, keywords = {Actins, Cell Membrane, Cluster Analysis, Integrins, Lipids, Membrane Proteins}, issn = {1744-6848}, doi = {10.1039/d3sm00809f}, author = {Singha, Tapas and Polley, Anirban and Barma, Mustansir} } @article {3574, title = {Phase Segregation of Passive Advective Particles in an Active Medium.}, journal = {Phys Rev Lett}, volume = {116}, year = {2016}, month = {2016 Feb 12}, pages = {068306}, abstract = {

Localized contractile configurations or asters spontaneously appear and disappear as emergent structures in the collective stochastic dynamics of active polar actomyosin filaments. Passive particles which (un)bind to the active filaments get advected into the asters, forming transient clusters. We study the phase segregation of such passive advective scalars in a medium of dynamic asters, as a function of the aster density and the ratio of the rates of aster remodeling to particle diffusion. The dynamics of coarsening shows a violation of Porod behavior; the growing domains have diffuse interfaces and low interfacial tension. The phase-segregated steady state shows strong macroscopic fluctuations characterized by multiscaling and intermittency, signifying rapid reorganization of macroscopic structures. We expect these unique nonequilibrium features to manifest in the actin-dependent molecular clustering at the cell surface.

}, issn = {1079-7114}, doi = {10.1103/PhysRevLett.116.068306}, author = {Das, Amit and Polley, Anirban and Rao, Madan} } @article {1019, title = {Diffusion of GPI-anchored proteins is influenced by the activity of dynamic cortical actin.}, journal = {Mol Biol Cell}, volume = {26}, year = {2015}, month = {2015 Nov 5}, pages = {4033-45}, abstract = {

Molecular diffusion at the surface of living cells is believed to be predominantly driven by thermal kicks. However, there is growing evidence that certain cell surface molecules are driven by the fluctuating dynamics of cortical cytoskeleton. Using fluorescence correlation spectroscopy, we measure the diffusion coefficient of a variety of cell surface molecules over a temperature range of 24-37{\textdegree}C. Exogenously incorporated fluorescent lipids with short acyl chains exhibit the expected increase of diffusion coefficient over this temperature range. In contrast, we find that GPI-anchored proteins exhibit temperature-independent diffusion over this range and revert to temperature-dependent diffusion on cell membrane blebs, in cells depleted of cholesterol, and upon acute perturbation of actin dynamics and myosin activity. A model transmembrane protein with a cytosolic actin-binding domain also exhibits the temperature-independent behavior, directly implicating the role of cortical actin. We show that diffusion of GPI-anchored proteins also becomes temperature dependent when the filamentous dynamic actin nucleator formin is inhibited. However, changes in cortical actin mesh size or perturbation of branched actin nucleator Arp2/3 do not affect this behavior. Thus cell surface diffusion of GPI-anchored proteins and transmembrane proteins that associate with actin is driven by active fluctuations of dynamic cortical actin filaments in addition to thermal fluctuations, consistent with expectations from an "active actin-membrane composite" cell surface.

}, issn = {1939-4586}, doi = {10.1091/mbc.E15-06-0397}, author = {Saha, Suvrajit and Lee, Il-Hyung and Polley, Anirban and Groves, Jay T and Rao, Madan and Mayor, Satyajit} } @article {864, title = {Live cell plasma membranes do not exhibit a miscibility phase transition over a wide range of temperatures.}, journal = {J Phys Chem B}, volume = {119}, year = {2015}, month = {2015 Mar 26}, pages = {4450-9}, abstract = {

Lipid/cholesterol mixtures derived from cell membranes as well as their synthetic reconstitutions exhibit well-defined miscibility phase transitions and critical phenomena near physiological temperatures. This suggests that lipid/cholesterol-mediated phase separation plays a role in the organization of live cell membranes. However, macroscopic lipid-phase separation is not generally observed in cell membranes, and the degree to which properties of isolated lipid mixtures are preserved in the cell membrane remain unknown. A fundamental property of phase transitions is that the variation of tagged particle diffusion with temperature exhibits an abrupt change as the system passes through the transition, even when the two phases are distributed in a nanometer-scale emulsion. We support this using a variety of Monte Carlo and atomistic simulations on model lipid membrane systems. However, temperature-dependent fluorescence correlation spectroscopy of labeled lipids and membrane-anchored proteins in live cell membranes shows a consistently smooth increase in the diffusion coefficient as a function of temperature. We find no evidence of a discrete miscibility phase transition throughout a wide range of temperatures: 14-37 {\textdegree}C. This contrasts the behavior of giant plasma membrane vesicles (GPMVs) blebbed from the same cells, which do exhibit phase transitions and macroscopic phase separation. Fluorescence lifetime analysis of a DiI probe in both cases reveals a significant environmental difference between the live cell and the GPMV. Taken together, these data suggest the live cell membrane may avoid the miscibility phase transition inherent to its lipid constituents by actively regulating physical parameters, such as tension, in the membrane.

}, issn = {1520-5207}, doi = {10.1021/jp512839q}, author = {Lee, Il-Hyung and Saha, Suvrajit and Polley, Anirban and Huang, Hector and Mayor, Satyajit and Rao, Madan and Groves, Jay T} } @article {881, title = {Transbilayer lipid interactions mediate nanoclustering of lipid-anchored proteins.}, journal = {Cell}, volume = {161}, year = {2015}, month = {2015 Apr 23}, pages = {581-94}, abstract = {

Understanding how functional lipid domains in live cell membranes are generated has posed a challenge. Here, we show that transbilayer interactions are necessary for the generation of cholesterol-dependent nanoclusters of GPI-anchored proteins mediated by membrane-adjacent dynamic actin filaments. We find that long saturated acyl-chains are required for forming GPI-anchor nanoclusters. Simultaneously, at the inner leaflet, long acyl-chain-containing phosphatidylserine (PS) is necessary for transbilayer coupling. All-atom molecular dynamics simulations of asymmetric multicomponent-membrane bilayers in a mixed phase provide evidence that immobilization of long saturated acyl-chain lipids at either leaflet stabilizes cholesterol-dependent transbilayer interactions forming local domains with characteristics similar to a liquid-ordered (lo) phase. This is verified by experiments wherein immobilization of long acyl-chain lipids at one leaflet effects transbilayer interactions of corresponding lipids at the opposite leaflet. This suggests a general mechanism for the generation and stabilization of nanoscale cholesterol-dependent and actin-mediated lipid clusters in live cell membranes.

}, keywords = {Actins, Animals, Cell Membrane, CHO Cells, Cricetulus, Glycosylphosphatidylinositols, Lipid-Linked Proteins, Molecular Dynamics Simulation, Phosphatidylserines}, issn = {1097-4172}, doi = {10.1016/j.cell.2015.03.048}, author = {Raghupathy, Riya and Anilkumar, Anupama Ambika and Polley, Anirban and Singh, Parvinder Pal and Yadav, Mahipal and Johnson, Charles and Suryawanshi, Sharad and Saikam, Varma and Sawant, Sanghapal D and Panda, Aniruddha and Guo, Zhongwu and Vishwakarma, Ram A and Rao, Madan and Mayor, Satyajit} }