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Biophysical studies of membrane proteins/peptides. Interaction with ...

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CONCLUSIONS<br />

Chapter VI – Clustering <strong>of</strong> PI(4,5)P2 in Fluid PC Bilayers.<br />

Domain enrichment <strong>of</strong> PI(4,5)P 2 is expected to occur in vivo. Nevertheless the<br />

process must require strong binding to agents (<strong>proteins</strong>) that present a strong affinity for<br />

incorporation in cholesterol enriched domains. Clustering <strong>of</strong> charged phospholipids in<br />

the gel state is certainly possible as energetic requirements resulting from packing<br />

restrictions in this state can surpass the energetic penalty resulting from electrostatic<br />

repulsion between the clustered lipids. However, the same type <strong>of</strong> phenomena is<br />

expected to be highly unlikely in fluid bilayers, where packing restrictions are not so<br />

severe.<br />

The results from this study allowed us to conclude that PI(4,5)P 2 is homogeneously<br />

distributed in fluid PC bilayers. FRET <strong>studies</strong> ruled out the existence <strong>of</strong> any large scale<br />

domain (dimensions larger than 40 Å) enriched in PI(4,5)P 2 , while energy migration<br />

and fluorescence self-quenching <strong>studies</strong> <strong>of</strong> a fluorescently labelled PI(4,5)P 2 clearly<br />

indicate absence <strong>of</strong> strong short-range clustering <strong>of</strong> the lipid.<br />

Another important result from this study are the differences in water/bilayer<br />

partition efficiencies obtained for different protonation states <strong>of</strong> PI(4,5)P 2 . One possible<br />

explanation for this observation is that the micellar structure <strong>of</strong> PI(4,5)P 2 is destabilized<br />

at the completely deprotonated state (at low lamellar lipid concentrations, a micellar<br />

population <strong>of</strong> PI(4,5)P 2 is expected to be in equilibrium <strong>with</strong> a bilayer inserted<br />

population). These differences in partition behaviour can result in erroneous<br />

rationalization <strong>of</strong> fluorescent data, specially taking into account that the most popular<br />

fluorescently labelled PI(4,5)P 2 in the market presents very short acyl-chains, and will<br />

present even smaller partition efficiencies that the ones determined here for a long chain<br />

fluorescently labelled PI(4,5)P 2 .<br />

187

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