Ulk4 promotes Shh signaling by regulating Stk36 ciliary localization and Gli2 phosphorylation

The Hedgehog (Hh) family of secreted proteins governs embryonic development and adult tissue homeostasis through the Gli family of transcription factors. Gli is thought to be activated at the tip of primary cilium, but the underlying mechanism has remained poorly understood. Here, we show that Unc-51-like kinase 4 (Ulk4), a pseudokinase and a member of the Ulk kinase family, acts in conjunction with another Ulk family member Stk36 to promote Gli2 phosphorylation and Hh pathway activation. Ulk4 interacts with Stk36 through its N-terminal region containing the pseudokinase domain and with Gli2 via its regulatory domain to bridge the kinase and substrate. Although dispensable for Hh-induced Stk36 kinase activation, Ulk4 is essential for Stk36 ciliary tip localization, Gli2 phosphorylation, and activation. In response to Hh, both Ulk4 and Stk36 colocalize with Gli2 at ciliary tip, and Ulk4 and Stk36 depend on each other for their ciliary tip accumulation. We further show that ciliary localization of Ulk4 depends on Stk36 kinase activity and phosphorylation of Ulk4 on Thr1023, and that ciliary tip accumulation of Ulk4 is essential for its function in the Hh pathway. Taken together, our results suggest that Ulk4 regulates Hh signaling by promoting Stk36-mediated Gli2 phosphorylation and activation at ciliary tip.

Ci/Gli Phosphorylation by the Fused/Ulk Family Kinases

Hedgehog (Hh) signaling culminates in the conversion of the latent transcription factor Cubitus interruptus (Ci)/Gli from a repressor form (Ci^R/Gli^R) into an activator form (Ci^A/Gli^A). While sequential phosphorylation of Ci/Gli by protein kinase A(PKA), glycogen synthase kinase 3 (GSK3), and casein kinase 1 (CK1) is essential for its proteolytic processing that generates Ci^R/Gli^R, sequential phosphorylation of Ci/Gli by the Fused (Fu)/Unc-51 like kinase (Ulk) family kinases Fu/Ulk3/Stk36 and CK1 contributes to the formation of Ci^A/Gli^A. Fu/Ulk3/Stk36-mediated phosphorylation of Ci/Gli is stimulated by Hh, leading to altered interaction between Ci/Gli and the Hh pathway repressor Sufu. Here we describe both in vitro and in vivo assays that determine Ci/Gli phosphorylation by the Fu/Ulk family kinases and its regulation by Hh.

Gli Phosphorylation Code in Hedgehog Signal Transduction

Hedgehog (Hh) family of secreted proteins governs many key processes in embryonic development and adult tissue homeostasis in species ranging from insects to human. Deregulation of Hh signaling has been implicated in a wide range of human diseases including birth defect and cancer. Hh signaling pathway culminates in the conversion of the latent transcription factor Cubitus interruptus (Ci)/Gli from a repressor form (Ci^R/Gli^R) into an activator form (Ci^A/Gli^A). Both the production of Ci^R/Gli^R in the absence of Hh and the formation of Ci^A/Gli^A in response to Hh are regulated by phosphorylation. Whereas previous studies demonstrated that sequential phosphorylation by protein kinase A (PKA), glycogen synthase kinase 3 (GSK3), and casein kinase 1 (CK1) at multiple Ser/Thr clusters in the C-terminal region of Ci/Gli targets it for proteolytic processing to generate Ci^R/Gli^R, recent studies revealed that phosphorylation of Ci/Gli by the Fused (Fu)/Unc-51 like kinase (Ulk) family kinases Fu/Ulk3/Stk36 and other kinases contributes to Ci/Gli activation. Fu/Ulk3/Stk36-mediated phosphorylation of Ci/Gli is stimulated by Hh, leading to altered interaction between Ci/Gli and the Hh pathway repressor Sufu. Here we review our current understanding of how various Ci/Gli phosphorylation events are regulated and how they influence Hh signal transduction.

Atg1-independent induction of autophagy by the Drosophila Ulk3 homolog, ADUK

Although canonical autophagy regulation requires a multi-protein complex centered on the Ser/Thr-kinase Atg1 (mammalian Ulk1/2), alternative signals can induce autophagy independent of Atg1 through unknown mechanisms. Here we identify the Drosophila Ulk3 ortholog, another Drosophila Unc-51-like kinase (ADUK), as an Atg1-independent autophagy inducer. ADUK interacts with Atg1 complex members Atg13 and 200 kDa FAK family kinase-interacting protein, and requires Atg13 but not Atg1 for autophagy induction. Loss of ADUK shortens adult lifespan and reduces the autophagic response to a chemical stressor, dimethyl sulfoxide. However, ADUK is not required for autophagy induction by Atg1-dependent nutrient or developmental cues. Atg1 and ADUK/Ulk3 thus represent alternative catalytic components of a shared autophagy kinase complex.