Lateral membrane diffusion corralled.

Ever since the fluid mosaic membrane model was formulated in 1972 by Singer and Nicholson (1xThe fluid mosaic model of the structure of cell membranes. Singer, S.J. and Nicolson, G.L. Science. 1972; 175: 720–731Crossref | PubMedSee all References1), it has become increasingly clear that biological membranes are not only fluid, but also highly mosaic (2xMembranes are more mosaic than fluid. Engelman, D.M. Nature. 2005; 438: 578–580Crossref | PubMed | Scopus (436)See all References2). The large number of different lipid and protein species that are present in a biological membrane are not ideally mixed and associate, more or less permanently or only transiently, in clusters and domains of variable sizes (3xLine tensions, correlation lengths, and critical exponents in lipid membranes near critical points. Honerkamp-Smith, A.R., Cicuta, P...., and Keller, S.L. Biophys. J. 2008; 95: 236–246Abstract | Full Text | Full Text PDF | PubMed | Scopus (164)See all References, 4xControl of a nanoscopic-to-macroscopic transition: modulated phases in four-component DSPC/DOPC/POPC/Chol giant unilamellar vesicles. Konyakhina, T.M., Goh, S.L...., and Feigenson, G.W. Biophys. J. 2011; 101: L8–L10Abstract | Full Text | Full Text PDF | PubMed | Scopus (27)See all References, 5xRevitalizing membrane rafts: new tools and insights. Simons, K. and Gerl, M.J. Nat. Rev. Mol. Cell Biol. 2010; 11: 688–699Crossref | PubMed | Scopus (549)See all References, 6xQuantitative nanoscale analysis of IgE-FceRI clustering and coupling to early signaling proteins. Veatch, S.L., Chiang, E.N...., and Baird, B.A. J. Phys. Chem. B. 2012; 116: 6923–6935Crossref | PubMed | Scopus (16)See all References). This spatial and temporal heterogeneity is further modulated by interaction with the cytoskeleton on the inside and the glycocalyx on the outside of cell membranes. The effects of lateral and transverse membrane heterogeneity slow down the lateral diffusion of membrane lipids by one-to-two orders of magnitude; the lateral diffusion of membrane proteins can be even more affected by these complex interactions.In cells, effects of certain lipids or proteins on membrane diffusion are often studied by extracting or genetically eliminating one or more components. While this approach has proved to be extremely powerful for pinpointing contributions of individual components to membrane fluidity, these kinds of studies are limited by the extreme complexity of the systems that are investigated. Cholesterol extraction, for example, can have multiple indirect effects on the membrane probes that are monitored in diffusion experiments. Similarly, disrupting the cytoskeleton by pharmacological or mechanical treatments can greatly influence dynamical properties of the cell membrane through multiple mechanisms that are not easy to separate.In addition to these top-down elimination approaches in vivo, bottom-up reconstitution approaches in vitro have greatly influenced the molecular understanding of many complex systems in cell biology, including the structure and function of cellular membranes. In an article published in this issue, Heinemann et al. (7xLateral membrane diffusion modulated by minimal actin cortex. Heinemann, F., Vogel, S.K., and Schwille, P. Biophys. J. 2013; 104: 1465–1475Abstract | Full Text | Full Text PDF | PubMed | Scopus (26)See all References7) have taken a new reconstitution approach to investigate the lateral diffusion of lipids and peripheral lipid-linked proteins in model membranes that were bound to a minimal actin network. Lateral diffusion was shown to strongly depend on probe size (protein versus lipid) and mesh size of the coupled actin network. To obtain their data, Heinemann and Schwille (8xPreparation of micrometer-sized free-standing membranes. Heinemann, F. and Schwille, P. ChemPhysChem. 2011; 12: 2568–2571Crossref | PubMed | Scopus (18)See all References8) prepared free-standing lipid membranes over micrometer-size holes of an electron microscope grid. Diffusion of lipids (Atto-DOPE) and proteins (Alexa-cholera toxin B bound to gangliosides GM1) was measured by spot variation fluorescence correlation spectroscopy. Actin networks were assembled on both sides of the free-standing membranes by cross-linking a fraction of the actin subunits to the membrane via a biotin-neutravidin-biotin linkage. Natural variations in actin filament density were quantified in each spot by fluorescence intensity measurements of actin filament-binding Alexa-labeled phalloidin. Actin networks were additionally contracted by incubation with myosin filaments, which resulted in a further decrease of lateral diffusion of lipid and protein probes.Numerous interesting results emerge from this study that improves contemporary understanding of the coupling of membrane dynamics to a bound actin cortex:1.Reducing the mesh size of the cortex reduces the diffusion coefficients of lipids and membrane-bound proteins.2.Membrane-bound proteins with a larger size protruding into the actin cortex are more corralled and restricted in lateral motion than the smaller lipid probes.3.Fast and slow diffusion were observed simultaneously and contraction of the actin network by myosin filaments led to an increase of the slow diffusion component.4.Monte Carlo simulations rationalized these results in terms of diffusing species confined in small free areas of the network with a probability of crossing network boundaries that was greater for small lipid probes and smaller for large protein probes.In summary, the work reported in this article greatly expands our mechanistic understanding of lipid and protein diffusion in membranes that are tightly coupled to a dynamically variable cortex of cytoskeleton. It should now be possible to build further complexity into this system and study the interplay among lipid and protein composition (including components that produce lipid rafts) with the cytoskeletal cortex and find out how they combine to modulate membrane diffusion. It would also be interesting to further extend these studies to model membranes with asymmetric lipid, protein, and cytoskeletal cortex compositions (9xDomain coupling in asymmetric lipid bilayers. Kiessling, V., Wan, C., and Tamm, L.K. Biochim. Biophys. Acta. 2009; 1788: 64–71Crossref | PubMed | Scopus (100)See all References9).