The Contribution of Loops versus Transmembrane Helices

Is the manner in which transmembrane (TM) helices pack together dictated by the sequences contained within the membrane or by the extramembranous sequences, or loops There is now considerable evidence that most loops are not essential in specifying the fold of membrane proteins. First, in many cases, the TM helices can encode considerable information for specifying the fold. Many single TM helices, such as the TM helix from glycophorin A, self-associate in the absence of their extramembranous domains (Lemmon et al., 1992a,b). Second, there are many examples in which the loops between TM helices in 

Forces That Stabilize Transmem bra nf. Helix Interactions 

Lipid bilayers are complex environments and the activity of membrane proteins can be modulated by the overall properties of the lipid bilayer, as well as by specific interactions with individual lipids. Overall bilayer 

Hydrophobic matching is just one example in which a region of a molecule, that is, the protein, has chemical properties that match the properties of its surroundings, the lipids. With this view in mind, we should also consider how well the surface of the membrane protein matches the interfacial and the water-exposed regions of the bilayer. For example, aromatic residues have an affinity for the interface region (Wimley and White, 1996 Yau et al., 1998) and the positively charged residues have an affinity for the phosphate head groups. 

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). 

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