Self-association of Transmembrane α-Helices in Model Membranes

Abstract

Interactions between transmembrane helices play a key role in almost all cellular processes involving membrane proteins. We have investigated helix-helix interactions in lipid bilayers with synthetic tryptophan-flanked peptides that mimic the membrane spanning parts of membrane proteins. The peptides were functionalized with pyrene to allow the self-association of the helices to be monitored by pyrene fluorescence and Trp-pyrene fluorescence resonance energy transfer (FRET). Specific labeling of peptides at either their N or C terminus has shown that helix-helix association occurs almost exclusively between antiparallel helices. Furthermore, computer modeling suggested that antiparallel association arises primarily from the electrostatic interactions between ␣-helix backbone atoms. We propose that such interactions may provide a force for the preferentially antiparallel association of helices in polytopic membrane proteins. Helix-helix association was also found to depend on the lipid environment. In bilayers of dioleoylphosphatidylcholine, in which the hydrophobic length of the peptides approximately matched the bilayer thickness, association between the helices was found to require peptide/lipid ratios exceeding 1/25. Selfassociation of the helices was promoted by either increasing or decreasing the bilayer thickness, and by adding cholesterol. These results indicate that helix-helix association in membrane proteins can be promoted by unfavorable protein-lipid interactions.Most membrane proteins have one or more hydrophobic segments that span the membrane in an ␣-helical conformation. Interactions between these transmembrane (TM) 4 helices are important for determining the structure of multispanning membrane proteins and for assembly of membrane proteins into oligomeric structures (1-4). Several factors are thought to be responsible for the association of helices in membrane proteins, including surface complementarity, the presence of polar residues in the transmembrane region (5-7), and certain specific motifs such as the well known GXXXG pattern (8, 9). It is likely that several of these factors act in concert to determine the final folded structure, or the association of monomers to form an oligomer. In addition to helix-helix interactions, interactions between the helices and surrounding lipids also play a role in the organization and assembly of TM helices. For example, even when helices do not exhibit any tendency to undergo specific association (10 -12), helix-helix association could still occur as a result of poor packing between the lipids and helices, or from a favorable change in entropy resulting from the release of helix-bound lipids upon helix association. In these cases, helix association is primarily driven by lipid-protein interactions rather than strongly favorable protein-protein interactions. It is likely that in real membrane proteins the driving forces for folding involve both types of interaction, whether or not specific protein-protein recognition motifs are present.One property of a pr...

Potential of mean force analysis of the self-association of leucine-rich transmembrane α-helices: Difference between atomistic and coarse-grained simulations

“…4 Using a Trp-flanked poly-(Leu-Ala) stretch of variable length (WALP), Killian and co-workers showed that dimerization was almost exclusively antiparallel, not parallel. 18,20 Dimerization of helical peptides with a sequence of (AALALAA) 3 also exhibits dependency on the lipid tail length, 5 allowing for analysis of the contribution of enthalpy and entropy. In computational chemistry, many studies have analyzed TM helix dimerization of glycophorin A (GpA), 10,13,[21][22][23][24][25] whereas relatively few studies have analyzed peptides with simple sequences, except WALP and Lys-flanked poly-(Leu-Ala) (KALP) peptides.…”

“…16 On the other hand, analyses using TM peptides with simple sequences showed robust association of helices, even without specific recognition motifs. 17,18 Although many studies cannot be listed here, studies using electron spin resonance have reported that Leu-rich peptides are a) Author to whom correspondence should be addressed. Electronic mail:…”

Lipid mediated packing of transmembrane helices – a dissipative particle dynamics study

“…It is, however, well-known that the membrane plays an important role in determining the tilt of TM helices [13][14][15][16][17][18][19] and also in mediating the interactions between TM helices. 15,[20][21][22][23][24][25] These phenomena are driven by the hydrophobic mismatch, which is dened as the difference between the hydrophobic length of a helix and the hydrophobic thickness of the membrane it is embedded in. It is reasonable to expect that the membrane, and hence the hydrophobic mismatch, also plays a role in the helix-helix packing.…”

Structure, topology and assembly of a 32-mer peptide corresponding to the loop 3 and transmembrane domain 4 of divalent metal transporter (DMT1) in membrane-mimetic environments

“…Moreover, helix-helix association was also found to depend on the lipid environments in a modelmembrane system, e.g. the bilayer thickness and peptide/ lipid ratios [26]. Our present study indicated that even incorporation sequences of loops would affect aggregation states of transmembrane domains in micelles.…”

“…Interactions between transmembrane helices play a key role in almost all cellular processes involving membrane proteins. Previous studies demonstrated that the association between transmembrane helices to form oligomers was promoted by unfavorable protein-lipid interactions [26]. Such association would lead to polar or charged residues in the membrane apolar core to be stabilized by ionic force, hydrogen bonds and the interstitial water molecules between polar residues in the interior of the membrane helix bundle [27].…”