Lipids segregate with each other into small domains in biological membranes, which can facilitate the associations of particular proteins. The segregation of cholesterol and sphingomyelin (SPM) into domains known as rafts is thought to be especially important. The formation of rafts was studied by using planar bilayer membranes that contained rhodamine-phosphatidylethanolamine (rho-DOPE) as a fluorescent probe, and wide-field fluorescence microscopy was used to detect phase separation of the probe. A fluorescently labeled GM(1), known to preferentially partition into rafts, verified that rho-DOPE faithfully reported the rafts. SPM-cholesterol domains did not form at high temperatures but spontaneously formed when temperature was lowered to below the melting temperature of the SPM. Saturated acyl chains on SPMs therefore promote the formation of rafts. The domains were circular (resolution > or = 0.5 microm), quickly reassumed their circular shape after they were deformed, and merged with each other to create larger domains, all phenomena consistent with liquid-ordered (l(o)) rather than solid-ordered (s(o)) domains. A saturated phosphatidylcholine (PC), disteoryl-PC, could substitute for SPM to complex with cholesterol into a l(o)-domain. But in the presence of cholesterol, a saturated phosphatidylethanolamine or phosphatidylserine yielded s(o)-domains of irregular shape. Lipids with saturated acyl chains can therefore pack well among each other and with cholesterol to form l(o)-domains, but domain formation is dependent on the polar headgroup of the lipid. An individual raft always extended through both monolayers. Degrading cholesterol in one monolayer with cholesterol oxidase first caused the boundary of the raft to become irregular; then the raft gradually disappeared. The fluid nature of rafts, demonstrated in this study, may be important for permitting dynamic interactions between proteins localized within rafts.

Electric fields promote pore formation in both biological and model membranes. We clamped unmodified planar bilayers at 150-550 mV to monitor transient single pores for a long period of time. We observed fast transitions between different conductance levels reflecting opening and closing of metastable lipid pores. Although mean lifetime of the pores was 3 +/- 0.8 ms (250 mV), some pores remained open for up to approximately 1 s. The mean amplitude of conductance fluctuations (approximately 500 pS) was independent of voltage and close for bilayers of different area (40,000 and 10 microm(2)), indicating the local nature of the conductive defects. The distribution of pore conductance was rather broad (dispersion of approximately 250 pS). Based on the conductance value and its dependence of the ion size, the radius of the average pore was estimated as approximately 1 nm. Short bursts of conductance spikes (opening and closing of pores) were often separated by periods of background conductance. Within the same burst the conductance between spikes was indistinguishable from the background. The mean time interval between spikes in the burst was much smaller than that between adjacent bursts. These data indicate that opening and closing of lipidic pores proceed through some electrically invisible (silent) pre-pores. Similar pre-pore defects and metastable conductive pores might be involved in remodeling of cell membranes in different biologically relevant processes.

[1] The global BATS-R-US MHD code is used to simulate the interaction of a moderately strong interplanetary shock with the Earth's magnetosphere. The model predicts the propagation of a transmitted fast shock through the magnetosheath and magnetosphere and the reflection of this shock from the inner numerical boundary. The reflected fast shock propagates sunward through the dayside magnetosphere and magnetosheath. The passage of the transmitted shock causes the bow shock and magnetopause to move inward, while the passage of the reflected fast shock causes these boundaries to move outward, consistent with previously reported in situ observations. A supplementary study employing a one-dimensional MHD model addresses the interaction of the forward fast shock with the plasmapause. This study demonstrates that most of the energy associated with the fast shock energy penetrates into the plasmasphere. Consequently, the transmitted fast wave must reach the ionosphere, produce the well known sudden impulse signatures, and then be reflected due to the shielding effects of ionospheric currents. The predictions of the numerical simulations are consistent with observations of the dayside geosynchronous magnetic field.

Tau is a major microtubule-associated protein which induces bundling and stabilization of axonal microtubules (MTs). To investigate the interaction of tau with MTs in living cells, we expressed GFP-tau fusion protein in cultured Xenopus embryo neurons and performed time-lapse imaging of tau-labeled MTs. Tau uniformly labeled individual MTs regardless of their assembly/disassembly status and location along the axon. Photobleaching experiments indicated that interaction of tau with MTs is very dynamic, with a half-time of fluorescence recovery of the order of 3 seconds. Treatment of cells with taxol, a drug that suppresses MT dynamics, rapidly induced detachment of tau from MTs. Although binding of tau to straight MTs was uniform, there was a heightened concentration of tau at the sites of high MT curvature. Our results suggest that dynamic interaction of tau with MTs may modify local mechanical properties of individual MTs and play a crucial role in the remodeling of the MT cytoskeleton during neuronal plasticity.