Monomeric solution structure of the prototypical 'C' chemokine lymphotactin

Lymphotactin, the sole identified member of the C class of chemokines, spec ifically attracts T lymphocytes and natural killer cells. This 93-residue p rotein lacks 2 of the 4 conserved cysteine residues characteristic of the o ther 3 classes of chemokines and possesses an extended carboxyl terminus, w hich is required for chemotactic activity. We have determined the three-dim ensional solution structure of,recombinant human lymphotactin by NMR spectr oscopy. Under the conditions used for the structure determination, lymphota ctin was predominantly monomeric; however, pulsed field gradient NMR selfdi ffusion measurements and analytical ultracentrifugation revealed evidence o f dimer formation. Sequence-specific chemical shift assignments were determ ined through analysis of two- and three-dimensional NMR spectra of N-15- an d C-13/N-15-enriched protein samples. Input for the torsion angle dynamics calculations used in determining the structure included 1258 unique NOE-der ived distance constraints and 60 dihedral angle constraints obtained from c hemical-shift-based searching of a protein conformational database. The ens emble of 20 structures chosen to represent the structure had backbone and h eavy atom rms deviations of 0.46 +/- 0.11 and 1.02 +/- 0.14 Angstrom, respe ctively. The results revealed that human lymphotactin adopts the conserved chemokine fold, which is characterized by a three-stranded antiparallel bet a -sheet and a C-terminal a-helix. Two regions are dynamically disordered a s evidenced by H-1 and C-13 Chemical shifts and {N-15}- NOES: residues 1-9 of the amino terminus and residues 69-93 of the C-terminal extension. A fun ctional role for the C-terminal extension, which is unique to lymphotactin, remains to be elucidated.