• Cell membranes are composed of phospholipids, sterols, and proteins.

• Phospholipids are the scaffold in which other components are embedded.

• Cholesterol insertion is required to establish proper membrane permeability and fluidity.

Cholesterol transport rates in model membranesS. Garg1, A.C. Woodka2, P.D. Butler2, U. Perez-Salas1,3, L. Porcar4,5

1ANL, 2NCNR, 3University of Illinois at Chicago, 4ILL, 5University of Delaware DMR-0944772, DE-AC02-06CH11357

Cell Membrane

Although proper cholesterol transport is essential to healthy cellular activity it is not very well understood. Transfer rates reported in the literature vary from fractions of a second to hours. In many cases the cholesterol is labeled with a fluorescent dye or substituted with a “cholesterol-like” molecule in order to make measurements possible. It turns out that these extraneous compounds can have a tremendous affect on the measured rates.

Two vesicle populations are made: cholesterol-rich donordonor vesicles and cholesterol-free acceptoracceptor vesicles.

Using the CHRNS-supported Small Angle Neutron Scattering (SANS) instrument, and contrast matching contrast matching we are able to measure cholesterol transfer rates without ANY extraneous compounds.

T1/2_f(min) T1/2_ex(min)Cholesterol 200 ± 13 88 ± 2

Chol w/ Cyclo 33 ± 3 12.5 ± 0.6DHE 22 ± 7 21 ± 2

Nature News doi:10.1038/news.2011.424 S. Garg et. al. Biophys. J. (2011) 101, 370-377

• The scattering intensity decreases by 50%50% as cholesterol equilibrates between donordonor and acceptor acceptor vesicles.

• Additives (Cylcodextrin) or mimics (DHEDHE) significantly change the measured kinetics.

OR?

Acceptor:lipid

Donor:Cholesterol

and lipidkex

• Cholesterol flipping is significantly slower than reported in the literature. • Flipping could be the rate limiting step for transport.• Cholesterol additives and substitutes alter flipping

and exchange rates.

Dan Neumann, National Institute of Standards and Technology, DMR 0944772