| Abstract
| - Fractalkine, a novel CX3C chemokine, is unusual because of both its membrane-associatedstructure and its direct role in cell adhesion. We have solved the solution structure of the chemokinedomain of fractalkine (residues 1−76) by heteronuclear NMR methods. The 20 lowest energy structuresin the ensemble have an average backbone rmsd of 0.43 Å, excluding the termini. In contrast to manyother chemokines which form homodimers, fractalkine's chemokine module is monomeric. Comparisonof the structure to CC and CXC chemokines reveals interesting differences which are likely to be relevantto receptor binding. These include a bulge formed by the CX3C motif, the relative orientation of theN-terminus and 30's loop (residues 30−38), and the conformation of the N-loop (residues 9−19). 15Nbackbone relaxation experiments indicate that these same regions of the protein are dynamic. We alsotitrated 15N-labeled protein with a peptide from the N-terminus of the receptor CX3CR1 and confirmedthat this region of the receptor contacts the fractalkine chemokine domain. Interestingly, the binding sitemaps roughly to the regions of greatest flexibility and structural variability. Together, these data providea first glimpse of how fractalkine interacts with its receptor and should help guide mutagenesis studies tofurther elucidate the molecular details of binding and signaling through CX3CR1.
|
