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

INTRODUCTION: LIPID-PROTEIN INTERACTIONS
However, this proposed mechanism for sorting of membrane proteins is still
controversial (Mouritsen, 2005).
Concerning membrane protein partition to specific regions of the bilayer, of
particular interest is the possibility that certain membrane proteins prefer insertion in the
boundaries between lipid phases in the membranes. These boundaries present a higher
probability of lipid packing defects that can be used by certain proteins to access deeper
regions in the membrane (Pérez-Gil et al., 2005).
2.10. Lipid sorting by proteins and formation of
lipid domains
The presence of very stable lipid domains in the membrane is only expected if the
free energies for interaction between phase separated lipids is very high. In
biomembranes this is rarely the case, and therefore lateral structures that arise must have
a reasonably dynamical nature. In model membranes, the differences in free-energies
for lipid-lipid interactions are typically on the order of a few calories per mole. In
contrast, the free-energies involved in protein-lipid interactions are much larger
(Hinderliter et al., 2001). Therefore membrane proteins when incorporated in lipid
membranes can contribute decisively to the final lipid phase equilibria.
In the presence of a multicomponent lipid mixture in the fluid phase, insertion of a
membrane protein can lead, as already described, to enrichment of one of the lipid
species around the protein due to more efficient packing in the protein-lipid interface.
This preferential association can stabilize a lipid phase around the protein, even under
conditions where it is metastable in the protein-free system. This preferred phase is said
to wet the protein (Gil et al., 1998). The wetting layer of lipids only extends over a
finite (nonmacroscopic) distance. Theoretical studies have predicted that effects on
lipids by inclusion of proteins may extend up to 20 lipid shells around the protein (Fattal
and Ben-Shaul, 1993; Sperotto and Mouritsen, 1991), and results from some
experimental studies were rationalized on the basis of wetting of membrane proteins and
formation of lipid phases induced by protein-lipid interactions (Lehtonen and Kinnunen,
1997; Dumas et al., 1997; Hinderliter et al, 2004).
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