Phosphorylation-induced SUMOylation promotes Ulk4 condensation at the ciliary tip to transduce Hedgehog signal.

Abstract

Hedgehog (Hh) signaling controls embryonic development and adult tissue homeostasis through the Gli family of transcription factors. In vertebrates, Hh signal transduction depends on the primary cilium, where Gli proteins are thought to be activated at the ciliary tip, but the underlying mechanism has remained poorly understood. Here, we provide evidence that two Unc-51-like kinase (Ulk) family members, Stk36 and Ulk4, regulate Gli2 ciliary tip localization and activation through phosphorylation and SUMOylation-mediated condensation in response to the Hh family protein Shh. We find that Stk36-mediated phosphorylation of Ulk4 promotes its SUMOylation in response to Shh, and the subsequent interaction between SUMO and a SUMO-interacting-motif (SIM) in the C-terminal region of Ulk4 drives Ulk4 self-assembly to form biomolecular condensates that also recruit Stk36 and Gli2. SUMOylation or SIM-deficient Ulk4 failed to accumulate at ciliary tip to activate Gli2 whereas phospho-mimetic mutation of Ulk4 sufficed to drive Ulk4, Stk36 and Gli2 condensation at ciliary tip, leading to constitutive Shh pathway activation in a manner dependent on Ulk4 SUMOylation. Taken together, our results suggest that phosphorylation-dependent SUMOylation of Ulk4 promotes kinase-substrate condensation at ciliary tip to transduce the Hh signal.