The role of transmembrane segment II in 7TM receptor activation

During the two past decades tremendous effort has been put into uncovering the activation mechanism of 7TM receptors. The majority of such studies have focused on the major binding pocket, comprised of transmembrane segments (TM) -III through -VII, as most non-peptide and peptide ligands, in addition to biogenic amines and retinal a.m.o. bind to residues in this region. Consequently the major helical movements occur here during activation, as described recently in the Global Toggle Switch Model for Family A (also known as rhodopsin-like) members of the 7TM receptors. As a result, the minor binding pocket, comprised of TM-I, -II and, in part, -III and -VII, has received much less attention. With a few exceptions, such as the highly conserved Asp in position II:10/2.50, the residues in this region have generally been considered insignificant with regard to receptor activation. However, accumulating evidence emphasize that this is not the case. In this review, we focus on TM-II with an emphasis on position II:20/2.60, and present data from structure-activity studies on a range of Family A 7TM receptors including chemokine, ghrelin and melanocortin receptors in addition to the orphan EBI2 suggesting that TM-II has important functions for both ligand-dependent and -independent activation of 7TM receptors.