Helix-helix interactions inside lipid bilayers

Far from being simple hydrophobic anchors, it is now clear that the transmembrane α-helices of integral membrane proteins can participate in strong, specific interactions that are important in their folding and oligomerization. Crystallographic studies of 21 such helices have indicated that these interactions are similar to those described for soluble proteins. Helix-helix interactions are also important in the oligomerization of a number of proteins that have a single transmembrane α-helix. The interactions are rather specific, involving interhelical salt bridges, hydrogen bonds or precise packing interactions. In some cases, such oligomerization is required for exit from the endoplasmic reticulum. The transmembrane helices of some Golgi-residing proteins also contain sufficient information to ensure their retention in this compartment. Finally, interactions between transmembrane α-helices may be important in the mechanism of transmembrane signalling by a number of membrane-bound receptors.