Dynamin, the founding member of a family of dynamin-like proteins (DLPs) implicated in membrane remodelling, has a critical role in endocytic membrane fission events. The use of complementary approaches, including live-cell imaging, cell-free studies, X-ray crystallography and genetic studies in mice, has greatly advanced our understanding of the mechanisms by which dynamin acts, its essential roles in cell physiology and the specific function of different dynamin isoforms. In addition, several connections between dynamin and human disease have also emerged, highlighting specific contributions of this GTPase to the physiology of different tissues.

Dynamin, a membrane-remodelling GTPase

The membrane raft hypothesis postulates the existence of lipid bilayer membrane heterogeneities, or domains, supposed to be important for cellular function, including lateral sorting, signaling, and trafficking. Characterization of membrane lipid heterogeneities in live cells has been challenging in part because inhomogeneity has not usually been definable by optical microscopy. Model membrane systems, including giant unilamellar vesicles, allow optical fluorescence discrimination of coexisting lipid phase types, but thus far have focused on coexisting optically resolvable fluid phases in simple lipid mixtures. Here we demonstrate that giant plasma membrane vesicles (GPMVs) or blebs formed from the plasma membranes of cultured mammalian cells can also segregate into micrometer-scale fluid phase domains. Phase segregation temperatures are widely spread, with the vast majority of GPMVs found to form optically resolvable domains only at temperatures below ≈25°C. At 37°C, these GPMV membranes are almost exclusively optically homogenous. At room temperature, we find diagnostic lipid phase fluorophore partitioning preferences in GPMVs analogous to the partitioning behavior now established in model membrane systems with liquid-ordered and liquid-disordered fluid phase coexistence. We image these GPMVs for direct visual characterization of protein partitioning between coexisting liquid-ordered-like and liquid-disordered-like membrane phases in the absence of detergent perturbation. For example, we find that the transmembrane IgE receptor FceRI preferentially segregates into liquid-disordered-like phases, and we report the partitioning of additional well known membrane associated proteins. Thus, GPMVs now provide an effective approach to characterize biological membrane heterogeneities.

/pdf/large-scale-fluid-fluid-phase-separation-of-proteins-and-2yky3ta3an.pdf

Large-scale fluid/fluid phase separation of proteins and lipids in giant plasma membrane vesicles

http://radio.cuci.udg.mx/bch/ES/papers/MembraneModel_BBA_2013-epub.pdf

The Fluid—Mosaic Model of Membrane Structure: Still relevant to understanding the structure, function and dynamics of biological membranes after more than 40 years

Membrane deformation is necessary to generate endocytic vesicles, but the molecular mechanisms proposed to drive membrane bending are controversial. Stachowiak and Schmid et al. report that crowding of proteins at the membrane is sufficient to induce curvature in vitro.

Membrane bending by protein-protein crowding

Cell membranes contain a large variety of lipid types and are crowded with proteins, endowing them with the plasticity needed to fulfill their key roles in cell functioning. The compositional complexity of cellular membranes gives rise to a heterogeneous lateral organization, which is still poorly understood. Computational models, in particular molecular dynamics simulations and related techniques, have provided important insight into the organizational principles of cell membranes over the past decades. Now, we are witnessing a transition from simulations of simpler membrane models to multicomponent systems, culminating in realistic models of an increasing variety of cell types and organelles. Here, we review the state of the art in the field of realistic membrane simulations and discuss the current limitations and challenges ahead.

Computational Modeling of Realistic Cell Membranes

Membrane curvature of a biological cell is actively involved in various fundamental cell biological functions. It has been discovered that membrane curvature and binding of peripheral membrane proteins follow a symbiotic relationship. The exact mechanism behind this interplay of protein binding and membrane curvature has not yet been properly understood. To improve understanding of the mechanism, we study curvature sorting of proteins in a model system consisting of a tether pulled from a giant unilamellar vesicle using mechanical-thermodynamic models. The concentration of proteins bound to the membrane changes significantly due to curvature. This has also been observed in experiments by other researchers. We also find that there is a phase transition based on protein concentration and we discuss the coexistence of phases and stability of solutions. Furthermore, when sorting is favorable, the increase in protein concentration stabilizes the tether in the sense that less pulling force is required to maintain the tether. A similar mechanism may be in place, when motor proteins pull tethers from donor membranes.

https://link.aps.org/accepted/10.1103/PhysRevE.85.051906

Curvature sorting of proteins on a cylindrical lipid membrane tether connected to a reservoir.

http://www.personal.psu.edu/qud2/Res/Pre/zdd09jcp.pdf

A phase field model for vesicle-substrate adhesion

Lipid bilayer vesicles with fluid/fluid phase coexistence are promising model systems for biological cell membranes. They permit the investigation of the mechanical properties of lipid membranes that are important for understanding mechanistic aspects of biological membrane function. Comparison of experimentally obtained vesicle shapes to mechanical membrane theories has enabled us to determine the values of line tension and mean-curvature bending stiffness of liquid-ordered and -disordered membranes. An additional important parameter that controls membrane geometry is the spontaneous curvature, driven, for example, by peripheral protein coats or asymmetric lipid composition. We examine to what extent effects of differing spontaneous curvature and Gauss curvature stiffness between coexisting fluid phases can be distinguished by membrane shape analysis. We find that the equilibrium neck geometries of dumbbell-shaped vesicles and the shapes of vesicles close to budding are very similar for vesicles that differ in spontaneous curvature or Gauss curvature stiffness differences. However, the two parameters have qualitatively different influence on discontinuous budding transitions.

https://iopscience.iop.org/article/10.1209/0295-5075/86/48003/pdf

Neck geometry and shape transitions in vesicles with co-existing fluid phases: Role of Gaussian curvature stiffness vs. spontaneous curvature

Inextensional vibration of zig-zag single-walled carbon nanotubes using nonlocal elasticity theories

We use the method of multiple scales (MMS) to study small perturbations, governed by a parameter e, of a harmonic oscillator by a small term with a large delay. These systems differ significantly from others where small terms have $$\cal O(1)]$$
 delays; or an $$\cal O(1)]$$
 term has $$\cal O(1)]$$
 delay in a system near a Hopf bifurcation. Here, the slow flow in time e t depends strongly on e even at lowest order, and itself has an $$\cal O(1)]$$
 delay. The MMS has already been applied elsewhere for such systems, but only to first order and with attention restricted to periodic and quasiperiodic solutions. Here, we address transients as well as proceed to second order. The second order analysis holds unless a special resonance occurs (we assume it does not). Several numerical examples are presented. In each case, the slow flows are infinite-dimensional, show strong e-dependence, require significantly less computation time than the full solutions, yet agree well with the same.

S. Das

Papers

Curvature sorting of proteins on a cylindrical lipid membrane tether connected to a reservoir.

A phase field model for vesicle-substrate adhesion

Neck geometry and shape transitions in vesicles with co-existing fluid phases: Role of Gaussian curvature stiffness vs. spontaneous curvature

Inextensional vibration of zig-zag single-walled carbon nanotubes using nonlocal elasticity theories

Second Order Multiple Scales for Oscillators with Large Delay