Transduction of the Hedgehog Signal through the Dimerization of Fused and the Nuclear Translocation of Cubitus Interruptus

As one of the main morphogens during metazoan development, the graded Hedgehog (Hh) signal is transduced across the plasma membrane by Smoothened (Smo) through the differential phosphorylation of its cytoplasmic tail, leading to the activation of the Hh pathway and the differential expression of target genes. However, how Smo transduces the graded Hh signal via the Costal2 (Cos2)/Fused (Fu) complex remains poorly understood. Recently, a team of researchers, led by ZHAO Yun and ZHANG Lei, at the Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, provided direct evidence and present a model that the cells response to a Hh gradient by translating Smo phosphorylation information to Fu dimerization and Cubitus interruptus (Ci) nuclear localization information.

 

The Hh family of secreted proteins is essential for development in both vertebrates and invertebrates. Malfunction of Hh signaling causes congenital disorders and cancers, such as birth defects, basal cell carcinoma (BCC) and obesity in humans. Thus, the study of Hh signaling transduction is of great importance in understanding the mechanism of oncogenesis and obesity and how different thresholds of a morphogen can generate distinct development outputs. The Hh pathway is composed of a series of repressive reactions. In the absence of Hh, downstream gene transcription is blocked by the Hh receptor, Patched (Ptc), which inhibits the function of GPCR-like protein Smoothened (Smo). Under this condition, a large protein complex, which contains the kinesin-related protein Cos2 and the Ser/Thr kinase Fu, associates with the transcription factor Ci and other kinases including PKA, GSK3 and CKI. The full-length Ci (Ci155) is phosphorylated and processed to generate a truncated form of Ci, Ci75, which functions as a Hh pathway repressor to block downstream gene expression. In the presence of Hh, however, the binding of Hh to Ptc allows Smo to be activated, resulting in the transduction of the Hh signal through the Cos2/Fu complex. In this case, Ci155 is stabilized and translocates to the nucleus, where it acts as a transcriptional activator, turning on downstream gene expression. Therefore, a graded Hh signal could be translated into different transcriptional responses via Hh signaling components. However, more questions remain regarding Hh signaling cascade. For example, it is not known how the gradient information of an Hh signal is transduced through Smo and the Cos2/Fu complex to distinct downstream target genes. In addition, although many of the canonical Hh signal components are phosphorylated in Drosophila, including Smo, Cos2, Fu and Suppressor of Fused (Su(fu)), the specific role for these phosphorylation events in the regulation of Hh signal transduction remains poorly understood. For instance, it is poorly understood how the kinase activity of Fu is regulated and how activated Fu functions in this signaling cascade.

 

ZHANG Yanyan and her colleagues, under the supervision of Drs. ZHAO Yun and ZHANG Lei, found that Smo activation, different levels of which are induced by different Smo mutants, recruits the Cos2/Fu complex to the membrane through the interactions between Smo and Cos2, which further induces Fu dimerization. Dimerized Fu is auto-phosphorylated and transduces the Hh signal by phosphorylating Cos2 and Su(fu). This process promotes the dissociation of the full-length Ci and Cos2 or Su(fu), resulting in the translocation of Ci155 into the nucleus, which activates the expression of target genes. Their findings indicate that cells respond to Hh signaling through the regulation of Fu dimerization and its kinase activity.

 

This work entitled “Transduction of the Hedgehog signal through the dimerization of Fused and the nuclear translocation of Cubitus interruptus (Ci)” was published online in Cell Research on Aug 16^th, 2011.

 

This study was supported by the grants from the Chinese Academy of Sciences, the Ministry of Science and Technology, National Natural Science Foundation of China and Shanghai Municipal Commission for Science and Technology.

 

AUTHOR CONTACT:

ZHAO Yun; ZHANG Lei

Institute of Biochemistry and Cell Biology, Shanghai Institutes for Biological Sciences, Chinese Academy of Sciences, Shanghai, China

Phone:86-21-54921245; E-mail: yunzhao@sibs.ac.cn; RayZhang@sibs.ac.cn

 

Working model for Fu transducing Hh morphogen gradient
A, Cartoon shows Hh gradient. B, In the absence of Hh, Ptc inhibits the function of Smo, and Smo C-tails fold back to form a closed inactive conformation. In this condition, the full length Ci, Ci155, is phosphorylated by PKA, CKI and GSK3 through the mediation of Cos2/Fu complex, and degraded into the truncated form, Ci75, blocking the target genes expression. C, In the presence of low Hh condition, low level Smo phosphorylation triggers a conformational change that increases the proximity between Smo C-tails, promotes recruitment of Cos2/Fu complex by interaction between Smo C-tail and Cos2, resulting in the dimerization of Fu. Dimerized Fu triggers its auto-phosphorylation and phosphorylation of Cos2, resulting in dissociation of Ci155 from Cos2, but not Su(fu). Ci155 could not be phosphorylated and degraded, relieving the inhibition of dpp expression. D, In the presence of high Hh concentration, more Hh binds with Ptc, leading to Smo hyper-phosphorylation and tighter Fu dimer by interaction between Cos2 and Smo C-tail, resulting in the phosphorylation of Su(fu). Phosphorylated Su(fu) dissociates from Ci155, promoting Ci155 nuclear translocation and downstream gene expression. (Image provided by Dr. Yun Zhao and Dr. Lei Zhang)