I-309 is a member o
f the CC subclass o
f chemokines and is one o
f only three human chemokinesknown to contain an additional, third disul
fide bond. The three-dimensional solution structure o
f I-309was determined by
1H nuclear magnetic resonance spectroscopy and dynamic simulated annealing. Thestructure o
f I-309, which remains monomeric at high concentrations, was determined on the basis o
f 978experimental restraints. The N-terminal region o
f I-309 was disordered, as has been previously observed
for the CC chemokine eotaxin but not others such as MCP-1 and RANTES. This was
followed in I-309by a well-ordered region between residues 13 and 69 that consisted o
f a 3
10-helix, a triple-strandedantiparallel
![](/images/gi<font color=)
fchars/beta2.gi
f" BORDER=0 ALIGN="middle">-sheet, and
finally a C-terminal
![](/images/gi<font color=)
fchars/alpha.gi
f" BORDER=0>-helix. Root-mean-square deviations o
f 0.61 and 1.16 wereobserved
for the backbone and heavy atoms, respectively. A comparison o
f I-309 to eotaxin and HCC-2revealed a signi
ficant structural change in the C-terminal region o
f the protein. The
![](/images/gi<font color=)
fchars/alpha.gi
f" BORDER=0>-helix normallypresent in chemokines was terminated early and was
followed by a short section o
f extended strand.These changes were a direct result o
f the additional disul
fide bond present in this protein. An examinationo
f the I-309 structure will aid in an understanding o
f the speci
ficity o
f this protein with its receptor,CCR8.