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

INTRODUCTION: LIPID-PROTEIN INTERACTIONS
annulus were considerably restricted by the presence of protein, the immobilized
lipids/TM domains ratio should come larger that for a single α-helix. Likely, the
influence of the TM proteins on the annular lipids is much stronger than on lipids on the
exterior shells, and the effects on the latter are not detectable by ESR.
The lipid exchange rate at the interface of the protein body was determined to be in
the region of 10 7 s -1 , which is on the same order but significantly lower than the value
obtained for lipid diffusion in the absence of protein (Marsh, 1990). This means that
although temporarily restricted to the protein surface, the lipid molecules interacting
with the protein body are not immobile and are still in exchange with other lipids.
Of relevance is the fact that lipids display different properties when at the surface of
proteins. This indicates that optimal hydrophobic matching might not necessarily occur
when the hydrophobic thickness of the protein matches the hydrophobic thickness of the
unperturbed bilayer (Dumas et al., 1999).
2.8. Lipid selectivity at protein interfaces
For membranes containing multiple lipid species, the hydrophobic matching
principle can result in even more complex behaviour at the lipid-protein interface.
Different lipid species are expected to present different free energies for interaction with
the presented TM domain and as a result, the lipid composition around the protein can
be different from the bulk lipid composition (Figure I.18).
Selectivity of membrane proteins for lipids in specific phases was observed in
various cases. In the presence of a mixture of DLPC and DSPC, a combination of
experimental and theoretical approaches indicated that bacteriorhodopsin exhibited
preferential interaction with DLPC at low temperatures, when both lipids where in the
gel phase, and at the fluid phase, bacteriorhodopsin had a preference for long chain
DSPC (Dumas et al., 1997). DLPC and DSPC present a very large difference in
hydrophobic length of acyl-chains and strongly nonideal mixing. Consequently, it is not
surprising that the affinity of the protein for the lipids differs drastically, and that the
protein is preferentially solvated by the lipids that are the best hydrophobic match in
each phase. Still, selectivity of proteins for lipids in binary mixtures with a much more
ideal mixing is also experimentally verified (Lee, 2003). Nevertheless, deviation of a
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