The arrangement of the transmembrane helices in the secretin receptor family of G-protein-coupled receptors

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The members of the secretin receptor family of G-protein-coupled receptors share no significant sequence similarity to the more familiar rhodopsin-like family. However, multiple sequence alignment analysis reveals seven hydrophobic regions with significant α-helical periodicity. Residues that are likely to be buried on the interior of the helical bundle and others that are likely to contact the lipid bilayer are identified. A predicted arrangement of the helical bundle is described in which, by comparison with the arrangement in the rhodopsin family, helices 2 and 7 are more buried within the bundle while helix 3 is more exposed to the lipid bilayer.