Thermodynamics of Insertion and Self-Association of a Transmembrane Helix: A Lipophobic Interaction by Phosphatidylethanolamine

Abstract

Thermodynamic parameters for the insertion and self-association of transmembrane helices are important for understanding the folding of helical membrane proteins. The lipid composition of bilayers would significantly affect these fundamental processes, although how is not well understood. Experimental systems using model transmembrane helices and lipid bilayers are useful for measuring and interpreting thermodynamic parameters (ΔG, ΔH, ΔS, and ΔC(p)) for the processes. In this study, the effect of the charge, phase, acyl chain unsaturation, and lateral pressure profile of bilayers on the membrane partitioning of the transmembrane helix (AALALAA)(3) was examined. Furthermore, the effect of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine (POPE) on the thermodynamics for insertion and self-association of the helix in host membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) was investigated in detail. Interbilayer transfer of the helix monomer from POPC to POPC/POPE (1/1) bilayers was unfavorable (ΔG = +4.5 ± 2.9 kJ mol(-1) at 35 °C) due to an increase in enthalpy (ΔH = +31.1 ± 2.1 kJ mol(-1)). On the other hand, antiparallel dimerization of the helices in POPC/POPE (1/1) bilayers was enhanced compared with that in POPC bilayers (ΔΔG = -4.9 ± 0.2 kJ mol(-1) at 35 °C) due to a decrease in enthalpy (ΔΔH = -33.2 ± 1.5 kJ mol(-1)). A greater thickness of POPC/POPE bilayers only partially explained the observed effects. The residual effects could be related to changes in other physical properties such as higher lateral pressure in the hydrocarbon core in the PE-containing membrane. The origin of the enthalpy-driven "lipophobic" force that modulates the insertion and association of transmembrane helices will be discussed.

Thermodynamic and kinetic characterization of transmembrane helix association

“…7,25,[29][30][31] The value of DG asso is consistent with in vitro estimates of related polyalanine peptides. 49,50 At the ms-s time-scale, we observed a low population of dimers under physiological conditions, in agreement with previous in vivo results. Although the association rate, k on , is fast, the relatively high off rates result in low dimer percentages.…”

“…These results are consistent with the association constants obtained from in vitro measurements 49,50 and low dimer percentages seen in in vivo studies. 51 Our results thus appear to resolve a seemingly contradictory aspect of transmembrane helix association.…”

The dimerization equilibrium of a ClC Cl−/H+ antiporter in lipid bilayers

“…Surprisingly, the thermodynamic forces driving this assembly remain poorly understood, due to a shortage of experimental systems where reversible equilibrium association can be observed in membranes. Dimerization models of single-pass transmembrane (TM) helices have provided a tractable system for free-energy measurements in detergent micelles (Fleming et al, 1997; MacKenzie and Fleming, 2008), and recently, in lipid bilayers (North et al, 2006; Chen et al, 2010; Hong et al, 2010; Yano et al, 2011). However, the relatively small change in solvent accessible surface area upon dimerization (MacKenzie et al, 1997) limits their potential to study protein-specific van der Waals interactions and lipid-solvent-dependent effects, the two driving forces hypothesized to be major players within the membrane environment (Popot and Engelman, 1990; White and Wimley, 1999; Bowie, 2005).…”

Dynamic clustering regulates activity of mechanosensitive membrane channels

“…redAs we did not observed any MSC clustering due to pure membrane-mediated interactions, we chose a top-down strategy. We know that: (i) MSC aggregation has been reported in vitro [16], and in vivo while working with MSCs labelled with a small covalent dye [17], (ii) direct inter-protein interactions, such as polar and electrostatic interactions and packing of small apolar side chains [24][25][26][27][28][29], can lead to attractions of trans-membrane proteins [18]. In addition, local lipid phase separation around the protein can yield effective inter-protein attraction and protein aggregation.…”

“…Nevertheless, since membrane-mediated interactions due to hydrophobic mismatch have never been directly experimentally quantified, it is hard to assess their importance in driving MSC aggregation observed in experiments [16,17]. There is however a growing body of evidence that direct protein-protein interactions drive aggregation of transmembrane helices, and also stabilize helixhelix interactions within a single protein [24][25][26][27][28][29]. It is likely that the same forces could also drive weak helix-helix interactions between different proteins should they be found close to each other.…”