Regulating Migration: The migration of cells around the body is an important factor in cancer development and the establishment of infection. Movement is induced by small proteins called chemokines, and so for a specific function, migration is controlled by a relevant chemokine binding to its respective receptor. This family of receptors is known as guanine (G) protein–coupled receptors, which span cell membranes to mediate between external signals from chemokines and internal mechanisms. The chemokine receptor CXCR4 is implicated in many types of cancer and in infection, and Wu et al. (p. 1066 , published online 7 October; see the Report by Chien et al. ) report on a series of crystal structures obtained for CXCR4 bound to small molecules. In every case, the same homodimer structure was observed, suggesting that the interface is functionally relevant. These structures offer insights into the interactions between CXCR4 and its natural chemokine, as well as with the virus HIV-1.

Tweaking Dopamine Reception: Dopamine modulates many cognitive and emotional functions of the human brain by activating G protein–coupled receptors. Antipsychotic drugs that block two of the receptor subtypes are used to treat schizophrenia but have multiple side effects.Chienet al.(p.1091; see the Research Article byWuet al.) resolved the crystal structure of one receptor in complex with a small-molecule inhibitor at 3.15 angstrom resolution. Homology modeling with other receptor subtypes might be a promising route to reveal potential structural differences that can be exploited in the design of selective therapeutic inhibitors having fewer side effects.

During the past few years, crystallography of G protein–coupled receptors (GPCRs) has experienced exponential growth, resulting in the determination of the structures of 16 distinct receptors—9 of them in 2012 alone. Including closely related subtype homology models, this coverage amounts to approximately 12% of the human GPCR superfamily. The adrenergic, rhodopsin, and adenosine receptor systems are also described by agonist-bound active-state structures, including a structure of the receptor–G protein complex for the β₂-adrenergic receptor. Biochemical and biophysical techniques, such as nuclear magnetic resonance and hydrogen-deuterium exchange coupled with mass spectrometry, are providing complementary insights into ligand-dependent dynamic equilibrium between different functional states. Additional details revealed by high-resolution structures illustrate the receptors as allosteric machines that are controlled not only by ligands but also by ions, lipids, cholesterol, and water. This wealth of data is helping redefine our knowledge of how GPCRs recognize such a diverse array of ligands and how they transmit signals 30 angstroms across the cell membrane; it also is shedding light on a structural basis of GPCR allosteric modulation and biased signaling.

GPCR Close-Up: Structures of G protein–coupled receptors (GPCRs) determined in the past few years, have provided insight into the function of this important family of membrane proteins.Liuet al.(p.232) used a protein-engineering strategy to produce a stabilized version of the human A_2Aadenosine receptor (A_2AAR). The high-resolution structure reveals the position of about 60 internal waters, which suggests an almost continuous channel in the GPCR and can explain the allosteric effects of Na⁺on ligand binding and how cholesterol may contribute to GPCR stabilization.