Molecular determinants of receptor binding and signaling by the CX3C chemokine fractalkine.
Fractalkine/CX3CL1 is a membrane-tethered chemokine that functions as a chemoattractant and adhesion protein by interacting with the receptor CX3CR1. To understand the molecular basis for the interaction, an extensive mutagenesis study of fractalkine's chemokine domain was undertaken. The results reveal a cluster of basic residues (Lys-8, Lys-15, Lys-37, Arg-45, and Arg-48) and one aromatic (Phe-50) that are critical for binding and/or signaling. The mutant R48A could bind but not induce chemotaxis, demonstrating that Arg-48 is a signaling trigger. This result also shows that signaling residues are not confined to chemokine N termini, as generally thought. F50A showed no detectable binding, underscoring its importance to the stability of the complex. K15A displayed unique signaling characteristics, eliciting a wild-type calcium flux but minimal chemotaxis, suggesting that this mutant can activate some, but not all, pathways required for migration. Fractalkine also binds the human cytomegalovirus receptor US28, and analysis of the mutants indicates that US28 recognizes many of the same epitopes of fractalkine as CX3CR1. Comparison of the binding surfaces of fractalkine and the CC chemokine MCP-1 reveals structural details that may account for their dual recognition by US28 and their selective recognition by host receptors.
Mizoue
L S
LS
Department of Molecular and Cell Biology, University of California, Berkeley, California 94720, USA.
Sullivan
S K
SK
King
D S
DS
Kledal
T N
TN
Schwartz
T W
TW
Bacon
K B
KB
Handel
T M
TM
D000595:Amino Acid Sequence; D000818:Animals; D001120:Arginine / Q000737:chemistry; D019556:COS Cells; D002118:Calcium / Q000378:metabolism; D002460:Cell Line; D002478:Cells, Cultured; D054428:Chemokine CX3CL1; D020523:Chemokines, CX3C / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism*; D002633:Chemotaxis; D004305:Dose-Response Relationship, Drug; D000939:Epitopes; D004926:Escherichia coli / Q000378:metabolism; D006801:Humans; D007700:Kinetics; D008024:Ligands; D008239:Lysine / Q000737:chemistry; D009682:Magnetic Resonance Spectroscopy; D008565:Membrane Proteins / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism*; D008958:Models, Molecular; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D009154:Mutation; D009457:Neuroglia / Q000166:cytology; D010649:Phenylalanine / Q000737:chemistry; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D015398:Signal Transduction; D013050:Spectrometry, Fluorescence; D013997:Time Factors; D014162:Transfection
C515538:CX3CL1 protein, human; D054428:Chemokine CX3CL1; D020523:Chemokines, CX3C; D000939:Epitopes; D008024:Ligands; D008565:Membrane Proteins; D010649:Phenylalanine; D001120:Arginine; D008239:Lysine; D002118:Calcium