3.4 Mutation of surface cysteine by serine may improve the
stabilization of IL-18 DM1234 to its receptor
According to the result from IFN-γ induction assay, molecular dynamic
(MD) simulation was performed to investigate the role of substitution of
surface cysteine by serine on structure of IL-18, which may affect the
activity of this protein. After 120 nanoseconds of MD simulation with
physiological parameters, all structures (WT, DM and DM1234) were
visualized and aligned by VMD program to determine the difference in
structural conformation of each type of IL-18. The result showed that
the obvious conformational change of IL-18 DM1234 was observed at
α-helix as a part of binding site I when compared to IL-18 DM (Fig. 5A
and 5B). This binding site I contains methionine 33 (M33) and aspartate
35 (D35) identified as the important binding residues of IL-18 to IL-18
receptor α (Kato et al., 2003). Figure 5B demonstrated this alteration
led to the higher similarity of this region between IL-18 WT and IL-18
DM1234, which may improve the stabilization of IL-18 DM1234 by IL-18
receptor like the native structure. Importantly, when looked deeper into
the direction of amino acids in this area, we found that D35 changed its
direction closer to valine 125 (V125) and serine 127 (S127) of IL-18
receptor compared to IL-18 WT (Fig. 5C). Structural alignment revealed
that the distance between D35 of IL-18 DM1234 and V125 and S127 of
IL-18R were in range of 2.88-4.72 A˚ (Fig. 5B) while D35 of IL-18 WT
showed the higher distance with 5.28 and 5.95 A˚ between V125 and S127,
respectively (Fig. 5C). Moreover, D35 of IL-18 DM also showed the higher
distance of these molecular pairs (4.35 and 4.95 A˚ for V125 and S127,
respectively) when compared to IL-18 DM1234 (Fig. 5D). This result
suggested that the substitution of surface cysteine by serine may
contribute to the alteration of IL-18 DM1234 activity through
conformational change at binding site I.