Structural insight into the recognition between Sufu and fused in the Hedgehog signal transduction pathway

Hedgehog signaling plays a crucial role in embryogenesis and adult tissue homeostasis, and mutations of its key components such as Suppressor of fused (Sufu) are closely associated with human diseases. The Ser/Thr kinase Fused (Fu) promotes Hedgehog signaling by phosphorylating the Cubitus interruptus (Ci)/Glioma-associated oncogene homologue (Gli) family of transcription factors. Sufu associates with both Fu and Ci/Gli, but the recognition mechanism between Sufu and Fu remains obscure. Here, our structure of the N-terminal domain (NTD) of Drosophila Sufu (dSufu) in complex with the Sufu-binding site (SBS) of Fu reveals that both main-chain β sheet formation and side-chain hydrophobic interactions contribute to the recognition between Sufu and Fu, and point mutations of highly conserved interface residues eliminated their association. Structural comparison suggests that Fu and Ci/Gli bind on opposite sides of dSufu-NTD, allowing the formation of a Fu-dSufu-Ci ternary complex which facilitates the phosphorylation of Ci/Gli by Fu. Hence, our results provide insights into the Sufu-Fu recognition mechanism.