RESEARCH HIGHLIGHTS G PROTEIN-COUPLED RECEPTORS Insights into chemokine receptors A recent paper published in Science modelling (based on previously has reported the crystal structure of defined GPCR structures) would be the chemokine receptor CXCR4, a difficult. In addition, CXCR4 lacked G protein-coupled receptor (GPCR) several features previously thought to that has been implicated in cancer be common to many GPCRs, such as metastasis and HIV infection. This is a short alpha helix located at the start the first report of a GPCR structure of the carboxyl terminus. that is activated by a protein ligand, CXCR4 is thought to form and it shows considerable differences homo- and hetero-oligomers, and compared with other GPCRs with the structures elucidated in this study known crystal structures. corroborated this. However, CXCR4 The authors determined the crys- oligomers would form in a consider- tal structures of the human CXCR4 ably different way from previous in complex with two antagonists: the models of GPCR oligomerization. small molecule antagonist IT1t and That is, CXCR4 uses helices V and the cyclic peptide inhibitor CVX15. VII to interact and is stabilized by As expected, the overall structure of helices III and IV. Furthermore, the CXCR4 consisted of the canonical structure of CXCR4 supported the bundle of seven transmembrane ‘two site’ model of chemokine signal- helices that are typical of all GPCRs. ling, whereby a core sequence of a The most notable difference between chemokine ligand docks to the bind- CXCR4 and other characterized ing domain of the receptor, and the GPCRs (such as rhodopsin, the amino terminal domain of the ligand β2-adrenergic receptor and the A2A triggers signalling. adenosine receptor) was the different Hopefully, knowledge of the shape and the different location of structure of CXCR4 will provide the ligand binding site. The binding new insights into chemokine cavity was larger, more open and receptor–ligand interactions, such located closer to the extracellular as the interaction of CXCR4 with its surface. It was defined by side chains endogenous ligand CXCL12 (which from helices I, II, III and VII but, inhibits HIV-1 entry) and with the unlike other known GPCR–ligand HIV-1 glycoprotein gp120. complexes, made no contact with Charlotte Harrison helices IV, V and VI. ORIGINAL RESEARCH PAPER Structures There were also differences in the of the CXCR4 chemokine GPCR with small arrangement of the transmembrane molecule and cyclic peptide antagonists. Science 7 Oct 2010 (doi:10.1126/science.1194396) helices, suggesting that homology NATURE REVIEWS | DRUG DISCOVERY VOLUME 9 | DECEMBER 2010 © 2010 Macmillan Publishers Limited. All rights reserved.