Multisite interaction with Sufu regulates Ci/Gli activity through distinct mechanisms in Hh signal transduction.

The tumor suppressor protein Suppressor of fused (Sufu) plays a conserved role in the Hedgehog (Hh) signaling pathway by inhibiting Cubitus interruptus (Ci)/Glioma-associated oncogene homolog (Gli) transcription factors, but the molecular mechanism by which Sufu inhibits Ci/Gli activity remains poorly understood. Here we show that Sufu can bind Ci/Gli through a C-terminal Sufu-interacting site (SIC) in addition to a previously identified N-terminal site (SIN), and that both SIC and SIN are required for optimal inhibition of Ci/Gli by Sufu. We show that Sufu can sequester Ci/Gli in the cytoplasm through binding to SIN while inhibiting Ci/Gli activity in the nucleus depending on SIC. We also find that binding of Sufu to SIC and the middle region of Ci can impede recruitment of the transcriptional coactivator CBP by masking its binding site in the C-terminal region of Ci. Indeed, moving the CBP-binding site to an "exposed" location can render Ci resistant to Sufu-mediated inhibition in the nucleus. Hence, our study identifies a previously unidentified and conserved Sufu-binding motif in the C-terminal region of Ci/Gli and provides mechanistic insight into how Sufu inhibits Ci/Gli activity in the nucleus.