Hydrophobic mismatch

Ghebremariam, B., Sidorov, V. and Matile, S. (1999) Direct evidence for the importance of hydrophobic mismatch for cell membrane recognition. Tetrahedron Letters, 40,1445-8. 

Fig. 5. Responses of a TM protein to hydrophobic mismatch. The hydrophobic regions of a TM protein (black regions) may be too long for the lipid core, creating a mismatch. To help reduce this stress, the protein may change its tilt angle or undergo more favorable associations. The protein may associate with a specific lipid, with a different tail length or curvature, or with another protein to reduce the lipid-facing surface area.

A hydrophobic mismatch between a membrane protein and the surrounding lipids may create a lateral force that would pull membrane proteins together. A general theoretical description of this force, referred to as a lateral capillary force, has been presented by Kralchevsky and co-workers (Kralchevsky, 1997 Kralchevsky and Nagayama, 2000). Although experimental verification of this force for membrane proteins in a bilayer has not been demonstrated, the force can be observed in larger systems, such as 1.7 fim latex beads at an air/water interface, and would be expected to operate on membrane proteins (Kralchevsky, 1997). 

The entire thermodynamic system of the membrane and TM protein must be considered to understand how the protein and bilayer achieve their native state. We have summarized four of the mechanisms, hydrophobic matching, tilt angles, and specific protein/lipid and protein/protein interactions that are important in determining the stability (Fig. 5). Other important factors, such as the stability of lipid/lipid interactions, have been left out of our protein-centric view. We describe a hydrophobic mismatch as an unfavorable interaction that can be relieved by the other three processes, but we would expect all these properties of the system to interact. We could easily describe the same equilibria by saying that a strain in curvature is relieved by a hydrophobic mismatch or that strong protein/protein packing interactions might help relieve the hydrophobic mismatch or curvature stress. The complex interplay between all these interactions is at the heart of what determines membrane protein stability and will no doubt be difficult to quantify. 

Figure 3 Possible mechanisms of MS channel activation by bilayer deformation forces. Hydrophobic mismatch and bilayer curvature are considered as deformation forces of pressure-induced changes in the lipid bilayer causing conformational changes in MS channels as indicated by the example of MscL (13). These changes were studied experimentally by reconstituting purified MscL proteins in liposome bilayers prepared from synthetic phosphatidylcholine lipids of well-defined composition. The changes in functional properties were examined by the patch-clamp technique, whereas the structural changes were determined by EPR and FRET spectroscopy. (Reproduced from Reference 12, with permission).

Killian J A. Hydrophobic mismatch between proteins and lipids in membranes. Biochim Biophys Acta 1998 1376 401-115. 

Fig. 2 Schematic representation of potential changes in integral membrane protein structure that could be imposed by a micellar environment (left hand side of each panel), compared to the native structure in bilayers (right). Possible distortions include (a) micelle-induced curvature in the TM helix or amphipathic helix (b) monomeric detergent molecules bound to a solvent-exposed region, in this case an aqueous cavity close to the micelle surface (c) altered relative orientations of amphipathic vs TM helices (d) loss of tilt relative to other TM segments. In this scenario hydrophobic mismatch between the TM helix and micelle are minimized by distortions in micelle structure that allow hydrophobic protein surfaces to remain in the hydrophobic phase. In the bilayer environment hydrophobic mismatch induces tilt, favoring a non-zero inter-helical crossing angles...

These fields differ quite substantially in their theoretical description concentrations are scalar variables, orientations are vectors, and differential geometry is at heart a tensor theory but, aU of them are known to mediate interactions. For instance, the fact that proteins might prefer one lipid composition over another and thus aggregate [217-220] is central to an important mechanism attributed to lipid rafts. Tilt-mediated protein interactions have also been studied in multiple contexts [32, 33, 159, 221-223]. It is even possible to describe all these phenomena within a common language [224], using the framework of covariant surface stresses [154, 155, 157-161]. However, in the present review we will restrict the discussion to only two examples, both related to membrane elasticity in Sect. 3.1 we will discuss interactions due to hydrophobic mismatch, and in Sect. 3.2 we will look at interactions mediated by the large-scale curvature deformation of the membrane. 

One major source of membrane-protein interactions that has been discussed in the literature for many decades is hydrophobic mismatch [20-22, 24, 26, 28, 29, 226-233]. If the width of the hydrophobic transmembrane domain of a protein is larger than the thickness of the lipid bilayer, the system can respond in two ways either the protein tilts [234-236] or the membrane deforms [18, 23, 24]. Both responses have biologically relevant consequences. On the one hand, the... 

Strandberg E, Esteban-Martin S, Ulrich AS, Salgado J (2012) Hydrophobic mismatch of mobile transmembrane helices merging theory and experiments. Biochim Biophys Acta 1818 1242-1249... 

Park SH, Opella SJ (2005) Tilt angle of a trans-membrane helix is determined by hydrophobic mismatch. J Mol Biol 350 310-318... 

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Harzer U, Bechinger B (2000) Alignment of lysine-anchored membrane peptides tmder conditions of hydrophobic mismatch A CD, N and P solid-state NMR spectroscopy investigation. Biochemistry 39(13) 106-13114... 

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Schmidt U, Guigas G, Weiss M (2008) Cluster formation of transmembrane proteins due to hydrophobic mismatching. Phys Rev Lett 101 128104... 

Muhammad N, Dworeck T, Fioroni M, Schwaneberg U. Engineaing of the E. coti outer membrane protein FhuA to overcome the hydrophobic mismatch in thick polymeric membranes. J Nanobiotechnol 2011 9. 

In contrast to polymersomes, there are various models of planar membranes monolayers at the water-air interface, free-standing bilayers, and solid-supported membranes. The functionality of proteins in natural membranes strongly depends on their mobility in the matrix, and this is thus an essential prerequisite for artificial membranes to mimic the dynamic environment of biomembranes in order to serve as templates for biomolecules.Therefore, the building blocks forming a bio-inspired membrane need to possess high flexibility to compensate the hydrophobic mismatch between the size of the biomolecules, and the membrane thickness. Furthermore, a variety of membrane properties (thickness, polarity, and surface charge) have to be considered for the successful insertion/attach-ment of biomolecules. Decoration of polymer membranes with biomolecules, either on their surfaces or inside the bilayers, can be achieved by various approaches, such as physical adsorption, insertion, and covalent binding. Compared to physical immobilization of biomolecules on... 

Figure 6.4 Bio-inspired polymeric vesicle with reconstituted gA biopores. In contrast to lipid based vesicles, gA exhibits increased hydrophobic mismatch with the polymeric membrane (9-13 nm thickness) whilst preserving its ion exchange function.

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