Biological membranes are known to contain compositional heterogeneities, often termed rafts, with distinguishable composition and function, and these heterogeneities participate in vigorous transport processes. Membrane lipid phase coexistence is expected to modulate these processes through the differing mechanical properties of the bulk domains and line tension at phase boundaries. In this contribution, we compare the predictions from a shape theory derived for vesicles with fluid phase coexistence to the geometry of giant unilamellar vesicles with coexisting liquid-disordered (L(d)) and liquid-ordered (L(o)) phases. We find a bending modulus for the L(o) phase higher than that of the L(d) phase and a saddle-splay (Gauss) modulus difference with the Gauss modulus of the L(o) phase being more negative than the L(d) phase. The Gauss modulus critically influences membrane processes that change topology, such as vesicle fission or fusion, and could therefore be of significant biological relevance in heterogeneous membranes. Our observations of experimental vesicle geometries being modulated by Gaussian curvature moduli differences confirm the prediction by the theory of Juelicher and Lipowsky.

Direct measurements by fluorescence correlation spectroscopy of lateral diffusion coefficients of fluorescent lipid analogs in lipid bilaryer membranes indicate self-diffusion coefficients D greater than 10(-7) square centimeters per second for various lipid systems above their reported transition temperatures. Cholesterol in egg lecithin at mole ratio of 1 : 2 reduces D by about twofold, while retained hydrocarbon solvent can increase it by two- to threefold.

We report the first imaging of the spatial distributions of transmembrane potential changes induced in nonexcitable cells by applied external electric fields. These changes are indicated by the fluorescence intensity of a charge-shift potentiometric dye incorporated in the cell plasma membrane and measured by digital intensified video microscopy.

Strong steric interactions among proteins on crowded living cell surfaces were revealed by measurements of the equilibrium spatial distributions of proteins in applied potential gradients. The fraction of accessible surface occupied by mobile surface proteins can be accurately represented by including steric exclusion in the statistical thermodynamic analysis of the data. The analyses revealed enhanced, concentration-dependent activity coefficients, implying unanticipated thermodynamic activity even at typical cell surface receptor concentrations.

Diffusion of the complex consisting of low density lipoprotein (LDL) bound to its receptor on the surface of human fibroblasts has been measured with the help of an intensely fluorescent, biologically active LDL derivative, dioctadecylindocarbocyanine LDL (diI(3)-LDL). Fluorescence photobleaching recovery and direct video observations of the Brownian motion of individual LDL-receptor complexes yielded diffusion coefficients for the slow diffusion on cell surfaces and fast diffusion on membrane blebs, respectively. At 10°C, <20% of the LDLreceptor complex was measurably diffusible either on normal human fibroblasts GM-3348 or on LDL-receptor-internalization-defective J. D. cells GM-2408A. At 21 ° and 28°C, the diffusion coefficients of the LDL-receptor complex were 1.4 and 4.5 x 10 -11 cm2/s with diffusible fractions of ~75 and 60%, respectively, on both cell lines. The lipid analog nitrobenzoxadiazolephosphatidylcholine (NBD-PC) diffused in the GM-2408A cell membrane at 1.5 X 10 -8 cm2/sec at 22°C. On blebs induced in GM-2408A cell membranes, the diI(3)-LDL receptor complex diffusion coefficient increased to ~10 -9 cm2/s, thus approaching the maximum theoretical predictions for a large protein in the viscous lipid bilayer. Cytoskeletal staining of blebs with NBD-phallacidin, a fluorescent probe specific for F-actin, indicated that loss of the bulk of the F-actin cytoskeleton accompanied the release of the natural constraints on lateral diffusion observed on blebs. This work shows that the internalization defect of 1. D. is not due to immobilization of the LDL-receptor complex since its diffusibility is sufficient to sustain even the internalization rates observed in the native fibroblasts. Nevertheless, as with many other cell membrane receptors, the diffusion coefficient of the LDL-receptor complex is at least two orders of magnitude slower on native membrane than the viscous limit approached on cell membrane blebs where it is released from lateral constraints. However, LDUreceptor diffusion may not limit LDL internalization in normal human fibroblasts.Low density lipoprotein (LDL), like insulin and epidermal growth factor (EGF), is one of a class of proteins that binds to a specific high-affinity cell surface receptor (2, 3) and is then internalized by the cell at a clathrin-coated area of membrane, a coated pit (4, 5). In normal human fibroblasts, >70% of the LDL receptor (LDL-R) population appears localized in coated pits even before LDL binding occurs (6). Thus, it is unclear whether diffusion to a coated pit on the membrane by either the bare LDL receptor or LDL bound to the LDL receptor, the LDL-receptor complex (LDL-RC), is a necessary step in the regulation of cholesterol synthesis and degradation. In contrast, the insulin and EGF-receptor complexes are reported to diffuse and form dusters before internalization at coated 846 pits (8, 9). The mobility of a ligand-receptor complex can be studied by fluorescence photobleaching recovery (FPR) by monitoring the characteristics of the relaxation of ...