The crystal structure of the chemokine domain of fractalkine shows a novelquaternary arrangement

Fractalkine, or neurotactin, is a chemokine that is present in endothelial cells from several tissues, including brain, liver, and kidney. It is the o nly member of the CX3C class of chemokines. Fractalkine contains a chemokin e domain (CDF) attached to a membrane-spanning domain via a mucin-like stal k. However, fractalkine can also be proteolytically cleaved from its membra ne-spanning domain to release a freely diffusible form. Fractalkine attract s and immobilizes leukocytes by binding to its receptor, CX(3)CR1. The x-ra y crystal structure of CDF has been solved and refined to 2.0 Angstrom reso lution. The CDF monomers form a dimer through an intermolecular beta-sheet, This interaction is somewhat similar to that seen in other dimeric CC chem okine crystal structures. However, the displacement of the first disulfide in CDF causes the dimer to assume a more compact quaternary structure relat ive to CC chemokines, which is unique to CX3C chemokines, Although fractalk ine can bind to heparin in vitro, as shown by comparison of electrostatic s urface plots with other chemokines and by heparin chromatography, the role of this property in vivo is not well understood.