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1 Instituto Superior Técnico
* To whom correspondence should be addressed. E-mail: lianacsilva{at}ist.utl.pt.
Submitted on June 23, 2006
Revised on August 30, 2006
Accepted on 28 September 2006
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Abstract |
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4mol%) in raft model membranes with a lipid composition resembling that of cell membranes, i.e., composed of different molar ratios of an unsaturated glycerophospholipid, sphingomyelin and cholesterol (Chol) along a liquid disordered-liquid ordered tie-line was explored. The application of a fluorescence multi-probe and multi-parameter approach, together with multiple fluorescence resonance energy transfer (FRET) pairs, in the well characterized palmitoyl-oleoyl-phosphocholine (POPC)/palmitoyl-sphingomyelin (PSM)/Chol ternary mixture, revealed that low palmitoyl-ceramide (PCer) concentrations strongly changed both the biophysical properties and lipid lateral organization of the ternary mixtures in the low to intermediate Chol/PSM-, small raft size-range (< 25mol% Chol). For these mixtures, PCer recruited up to 3 PSM molecules for the formation of very small (~ 4 nm) and highly-ordered gel domains, which became surrounded by rafts (liquid ordered phase) when Chol/PSM content increased. However, the size of these rafts did not change, showing that PCer did not induce the formation of large platforms or the coalescence of small rafts. In the high Chol/PSM-, large raft domains-range (> 33mol% Chol), Chol completely abolished the effect of PCer by competing for PSM association. Lipid rafts govern the biophysical properties and lateral organization in these last mixtures.
Key Words: Ceramide, Cholesterol, FRET, Lipid nanodomains, Lipid rafts, Sphingomyelin
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