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Closing the Ring: A Fourth Extracellular Loop in Chemokine Receptors

Formation of an additional extracellular loop affects the ligand-binding properties and rigidity of G protein–coupled receptors. Chemokine receptors are heterotrimeric guanine nucleotide–binding protein (G protein)–coupled receptors (GPCR) that play fundamental roles in many physiological and pathological processes. Typically, these receptors form a seven-transmembrane helix bundle, which is stabilized by a disulfide bond bridging the top of the third transmembrane segment (TM3) and the second extracellular loop (ECL2). Resolution of the three-dimensional structures of the chemokine receptors CXCR1, CXCR4, and CCR5 revealed the existence of a second disulfide bridge that links the N terminus of the receptor to the top of the seventh transmembrane segment (TM7), thereby closing the receptor into a ring. An important consequence of this second disulfide bond is the formation of an additional extracellular loop, which shapes the entrance of the ligand-binding pocket and adds rigidity to the overall surface of the receptor. Here, we discuss the features of these “pseudo-loops,” the structural requirements for their formation, and the effects they may have on receptor function.