1-2-dipalmitoyl-sn-glycero-3-ethylphosphocholine and 1-2-oleoylphosphatidylcholine

To elucidate the consequences of the saturated-unsaturated nature of lipid surface films, monolayers formed by an equimolar mixture of 1-palmitoyl-2-oleyl-sn-glycero-3-phosphocholine (POPC) and 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) lipids are investigated in a wide range of surface pressures. As such mixtures share some features with naturally-occurring surfactants, for example the lung surfactant, the systems are studied at the temperature relevant for human body. All-atom molecular dynamics simulations and Langmuir trough experiments are employed. The binary lipid mixture is compared with the corresponding one-component systems. Atomistic-level alterations of monolayer molecular properties upon lateral compression are scrutinized. These involve elevation of lateral ordering of lipid chains, modulation of chain and headgroup orientation, and reduction of lipid hydration. The presence of the unsaturated POPC in the DPPC/POPC mixture reduces the liquid expanded-liquid condensed coexistence region and moderates the phase transition. Simulations predict that nanoscale lipid de-mixing occurs with small transient DPPC clusters emerging due to local fluctuations of the lateral lipid arrangement. A vertical sorting of lipids induced by lateral compression is also observed, with DPPC transferred toward the water phase. Both the conformational lipid alterations due to monolayer compression as well as the existence of lateral dynamic inhomogeneities of the lipid film are potentially pertain to dynamic and non-homogeneous lipid interfacial systems.

Topics: 1,2-Dipalmitoylphosphatidylcholine; Lipids; Molecular Conformation; Molecular Dynamics Simulation; Phosphatidylcholines

Myosin Va is an actin-based molecular motor responsible for transport and positioning of a wide array of intracellular cargoes. Although myosin Va motors have been well characterized at the single-molecule level, physiological transport is carried out by ensembles of motors. Studies that explore the behavior of ensembles of molecular motors have used nonphysiological cargoes such as DNA linkers or glass beads, which do not reproduce one key aspect of vesicular systems--the fluid intermotor coupling of biological lipid membranes. Using a system of defined synthetic lipid vesicles (100- to 650-nm diameter) composed of either 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) (fluid at room temperature) or 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) (gel at room temperature) with a range of surface densities of myosin Va motors (32-125 motors per μm(2)), we demonstrate that the velocity of vesicle transport by ensembles of myosin Va is sensitive to properties of the cargo. Gel-state DPPC vesicles bound with multiple motors travel at velocities equal to or less than vesicles with a single myosin Va (∼450 nm/s), whereas surprisingly, ensembles of myosin Va are able to transport fluid-state DOPC vesicles at velocities significantly faster (>700 nm/s) than a single motor. To explain these data, we developed a Monte Carlo simulation that suggests that these reductions in velocity can be attributed to two distinct mechanisms of intermotor interference (i.e., load-dependent modulation of stepping kinetics and binding-site exclusion), whereas faster transport velocities are consistent with a model wherein the normal stepping behavior of the myosin is supplemented by the preferential detachment of the trailing motor from the actin track.

Topics: 1,2-Dipalmitoylphosphatidylcholine; Animals; Biological Transport, Active; Membranes, Artificial; Mice; Myosin Heavy Chains; Myosin Type V; Phosphatidylcholines; Transport Vesicles

The differential miscibility of membrane lipids is thought to be the basis for the formation of dynamic microdomain assemblies in cell membranes known as membrane rafts. Because of their relevance to the existence of rafts, there has been much interest in recent years in model membrane systems that display coexisting liquid ordered (l(o)) and liquid disordered phases (l(d)), such as the ternary mixture composed of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and cholesterol. Carefully equilibrating the samples at well controlled temperatures allows us to use a quantitative confocal fluorescence microscopy approach to measure the area fractions of coexisting fluid phases in DOPC/DPPC/cholesterol mixtures. We can then compare the behaviour of a large population of unilamellar vesicles with the domain fractions deduced from (2)H NMR experiments. The fluorescence results are established for the first time to be in quantitative agreement with those obtained using (2)H NMR spectroscopy within the two phase region of the phase diagram. We are also able to describe fine details of the phase separation and the approach to equilibrium not previously reported, in particular the existence of small spots of l(o) phase at temperatures higher than that at which the samples display domain fluctuations. A better understanding of coexisting fluid phases in model systems will assist in interpreting the behaviour of rafts in more complex biological membranes.

Topics: 1,2-Dipalmitoylphosphatidylcholine; Cholesterol; Deuterium; Hot Temperature; Magnetic Resonance Spectroscopy; Membrane Microdomains; Microscopy, Confocal; Phase Transition; Phosphatidylcholines