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

amphipatic helices (see Section 2.3). Predictions derived from these methods allow the
design of synthetic peptides for detailed biophysical studies.
A number of experiments showed that synthetic peptides corresponding to the
alpha-helical TM segments of some proteins can assemble and substitute the native
proteins as well as their functions after independent folding, validating the approach of
studying isolated peptide segments (Marsh, 1996). Popot and Engelman (1990; 1993)
rationalized these results with the proposal of a two-stage model for membrane insertion
of the TM segments from a intrinsic membrane protein. In the first stage, alpha-helices
are independently folded and inserted in the lipid bilayer, while in the second stage,
alpha-helices interact and assemble in the final protein structure.
2.3. Amphipatic helix
Amphipatic helices are protein alpha-helices for which one face along the helical
axis is hydrophilic whereas the opposite face is hydrophobic. The amphipatic helix is a
common feature of biologically active polypeptides and proteins, such as hormones,
antibiotics, and venoms. In this way, this structure can play a large number of roles. A
function of great structural importance is the shielding of the hydrophobic interior of
proteins exposed to aqueous environments, while for membrane proteins the roles
played by amphipatic helices are diverse. In the case of peripheric proteins they can act
as anchors to the membrane by tight binding to the hydrocarbon/water interface of the
bilayer as the structure of this boundary is highly complementary to the amphipatic
helix. The amphipatic structure is also able to promote protein-protein interactions
between hydrophobic segments of helices. In some cases this allows amphipatic
peptides to create channels and pores in the membrane via aggregation into oligomeric
bundles for which the hydrophobic side faces the hydrocarbon chains of the lipids and
the hydrophilic faces of the helices are exposed to the inside. Thus delineating a pore in
the bilayer (Epand, 1993). Recent studies also suggest a role of amphipatic helices in
the modulation of membrane curvature (see chapter V).
Overall, the amphiphilic character is a much more important dictator of peptide
conformation than the specific characteristic of its constituent amino acids (Taylor,
1990). Parameters like the ratio of hydrophobic to hydrophilic amino acids as well as
26