Flt3-FL binding interface. (A) Close-up view of the Flt3-FL binding interface. FL is colored in green, Flt3_D3 in gray, and Flt3_D2 in orange. Residues that constitute the cytokine-receptor interface are shown as sticks protruding from spheres centered at their Cα positions. FL residues are colored in yellow and Flt3 residues are colored in green. (B) The unusual Flt3_D2-Flt3_D3 interface. Flt3_D2-D4 (Cα trace in red) is shown together with FL in ribbon representation (green). Residues at the hydrophobic interface are shown as black sticks. Disulfide bonds in Flt3_D2-D3 are shown as ball and sticks (yellow). (C) Structure-based alignment of diverse FL sequences revealing strict conservation of the PISSXF-segment (residues 10-15) within the N-terminal loop (colored in red). A complete alignment can be found in supplemental Figure 3. (D) Structural comparison of bound versus the unbound FL.