Speaker
Dr
DAVID BARLOW
(KING'S COLLEGE LONDON)
Description
The lateral organisation of lipids and proteins in the eukaryotic cell membrane has been the focus of intense research. Of special interest is how the lipid components of the membrane can be triggered to phase-separate and form highly dynamic, nanoscale structures known as “rafts” (1). It has been well-established that under physiological conditions in membranes composed of phospholipid (PL), cholesterol (Chol) and sphingolipid (SL), phase separation occurs forming distinct liquid ordered (Lo) domains containing Chol and the highly saturated, conformationally ordered SLs (2,3), “floating” within a bed of “liquid-like” unsaturated and conformationally disordered PLs in a liquid disordered (Ld) phase (4, 5). Significantly, recruitment of certain proteins within these lipid domains has led to the so-called “raft hypothesis” wherein rafts are seen to be key in a variety of biological processes such as cell signalling and membrane trafficking (6).
To date, there have been a number of fluorescence microscopy and X-ray scattering studies of raft formation that have been conducted on biologically relevant ternary lipid mixtures involving dioleoylphosphatidylcholine (DOPC), mixed with dipalmitoylphosphatidylcholine (DPPC) and Chol (7, 8). There have been no studies yet conducted, however, that have allowed a direct determination of the molecular architecture of these raft-forming systems.
In the studies reported here, we aimed to rectify this deficiency, using the technique of lamellar neutron diffraction to study the formation of rafts in lipid multilayers comprising 2:2:1 ternary mixtures of DPPC, DOPC, and Chol. The multilayers were studied as a function of temperature, and the experiments conducted on the D16 diffractometer at the Institut de Laue Langevin, Grenoble, France.
Diffraction patterns were successfully recorded for the lipid multilayers at 298 K, 293 K, and at 288 K, with a relative humidity of 80%. At 298 K, the diffraction patterns showed a single lipid phase with d-spacing of 53.8 Å. As the temperature was lowered to 288 K, two phases appeared with d-spacings of 53.9 Å and 59.2 Å. The diffraction patterns were recorded with varying D2O:H2O solvents, and the neutron scattering length density profiles for the system computed by Fourier summation using the calculated structure factors. The scattering length density profiles for the two phases existing at 288 K were successfully modelled assuming one phase comprising a Lo domain of DPPC with Chol, and the second phase comprising a Ld domain of DOPC.
References
(1) Longo & Blanchette (2010) Biochim Biophys Acta 1798:1357. (2) Simons and Ikonen (1997) Nature 387:569. (3) Fanani et al (2010) Chem Phys Lipids 163:594. (4) Brown (1998) J Cell Sci 111:1. (5) London (2002) Cur Opin Struct Biol 12:480. 6) Quinn (2010) Progr. Lipid Res. 49:390. (7) Keller & Veatch (2003) Biophys. J. 85:3074. (8) Mills et al (2008) Biophys. J. 95:682
Primary author
Dr
DAVID BARLOW
(KING'S COLLEGE LONDON)
Co-authors
Dr
BRUNO DEME
(ILL)
Ms
DELARAM AHMADI
(KING'S COLLEGE LONDON)
Prof.
JAYNE LAWRENCE
(KING'S COLLEGE LONDON)
Dr
KATHERINE THOMPSON
(BIRKBECK COLLEGE)
