Induced conformational change in human IL-4 upon binding of a signal-neutralizing DARPin

The crystal structure of DARPin 44C12V5 that neutralizes IL-4 signaling has been determined alone and bound to human IL-4. A significant conformational change occurs in the IL-4 upon DARPin binding. The DARPin binds to the face of IL-4 formed by the A and C α-helices. The structure of the DARPin remains virtually unchanged. The conformational changes in IL-4 include a reorientation of the C-helix Trp91 side chain and repositioning of CD-loop residue Leu96. Both side chains move by >9 Å, becoming buried in the central hydrophobic region of the IL-4:DARPin interface. This hydrophobic region is surrounded by a ring of hydrophilic interactions comprised of hydrogen bonds and salt bridges and represents a classical "hotspot." The structures also reveal how the DARPin neutralizes IL-4 signaling. Comparing the IL-4:DARPin complex structure with the structures of IL-4 bound to its receptors (Hage et al., Cell 1999; 97, 271-281; La Porte et al., Cell 2008, 132, 259-272), it is found that the DARPin binds to the same IL-4 face that interacts with the junction of the D1 and D2 domains of the IL-4Rα receptors. Signaling is blocked since IL-4 cannot bind to this receptor, which it must do first before initiating a productive receptor complex with either the IL-13α1 or the γc receptor.