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

thickness than mcp, the N-terminus of the protein shifted its position in order to
accommodate the differences in hydrophobic lengths of protein and lipid, but the C-
terminus remains at approximately the same position in the bilayer interface due to
strong anchoring (Meijer et al., 2001). Another adaptation of mcp to hydrophobic
mismatch is changing the tilt angle of the transmembrane domain, as well as the rotation
of the helix at its long axis so as to optimize the hydrophobic and electrostatic
interactions at the C-terminus (Koehorst et al., 2004).
Several questions concerning lipid-protein and protein-protein interactions of mcp
are still subject of debate. During the life-cycle of bacteriophage M13, the aggregation
state of mcp changes repeatedly. In the phage particle it is aggregated in the coat, and in
the membrane it is believed to be monomeric (Russel, 1991; Spruijt and Hemminga,
1991), although a structural dimer might be formed as an intermediate species during
phage disruption (Henry and Sykes, 1992; Stopar et al., 1997b; Stopar et al., 1998). The
mcp structure is predominantly α-helical when inserted in the membrane, but depending
on the protein purification and membrane reconstitution procedure a irreversibly
aggregated β-sheet conformation of the protein appeared (Hemminga et al., 1992). Also,
factors such as L/P ratio, phospholipid headgroup type, length and degree of
unsaturation of the acyl-chains of lipids used, and the ionic strength of the buffer were
shown to be able to influence the α-helical to β-sheet transition. It is found that saturated
lipid chains and very high protein concentration resulted inevitably in high levels of
irreversible β-sheet aggregation (Spruijt and Hemminga, 1991). Nevertheless, the
irreversible β-sheet conformation of mcp is expected to be an artefact resulting from
inappropriate conditions of protein purification and reconstitution, as α-helical structure
is necessary for insertion in the phage particle (Hemminga et al., 1992).
The lipid composition of the inner membrane of non infected E. coli is about 70% of
PE, 25% of PG and 5% cardiolipin. During the infection of E. coli by the M13
bacteriophage, the levels of anionic lipids in the cell membrane are slightly increased
and blocking of phage assembly resulted in significant increases in anionic lipid content
in the inner membrane of bacteria (Pluschke et al., 1978). This phenomenon might
indicate that anionic lipids are required to maintain a functional state of the membranebound
form of the protein (Hemminga et al., 1992).
The in situ aggregational state of α-helical mcp is however still a matter of
discussion, as reversible oligomeric alpha-helical mcp was shown to exist in some
conditions (Hemminga et al., 1992). In vitro, aggregation behaviour is even more likely
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