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Figure 7.
FIGURE 7. Comparison of IL-15 quaternary complex model and
IL-2 quaternary complex. The superimposed structures are shown
as ribbon representations and are colored as in Fig. 6A. Note
that the distinct conformation of the hA-hB loop of IL-15 places
this region of the ligand into the vicinity of the hC-hC'1 loop
of  [c] where unique
interactions can potentially take place. Also note the position
and length of hA of IL-15 and IL-2 quaternary complex interface.
IL-2 residues Leu-12, Glu-15, Leu-18, and Leu-19 from the
N-terminal portion of hA participate in interactions at a
three-way junction between IL-2·IL-2Rβ and [c] that
is suggested to mediate cooperative binding. Due to the shorter
hA of IL-15, it has no homologous residue to Leu-12 of IL-2.
Furthermore, IL-2 residues Glu-15, Leu-18, and Leu-19 have
diverged to Ile-3, Arg-6, and Tyr-7 in IL-15 making it unlikely
that a homologous set of interactions takes place at the
IL-15·IL-2Rβ/ [c] junction.
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