Protein kinase A and casein kinase I directly regulate Smoothened activity to promote Hedgehog signal transduction.
Hedgehog (Hh) signaling occurring through the seven-transmembrane receptor Smoothened (Smo) is central to animal development. However, little is known about the mechanism of Smo regulation. Now, two different groups report that Smo is dually regulated by protein kinase A (PKA) and casein kinase I (CKI) in response to Hedgehog signaling.
PKA has an established inhibitory role in the Hh pathway through the phosphorylation of full length latent transcription factor Cubitus interrruptus (Ci-155). This targets Ci-155 for proteolytic processing to generate the repressor Ci-75. Consistent with this, overexpression of constitutively active PKA catalytic subunit (mC*) inhibits the induction of Hh target genes. However, Jia et al. observed that modest expression of mC* in Drosophila wings activated the Hh pathway, resulting in the anterior expansion of the Hh targets patched (ptc) and engrailed(en). The same was true when Apionishev et al. co-expressed mC* with Smo. Conversely, inhibition of PKA through the expression of a mutant PKA regulatory subunit (R*) in wing discs blocked Hh-induced Smo accumulation and activity, suggesting that PKA might indeed promote pathway activation.
Jia et al. used point mutants to show that Smo is sequentially phosphorylated by PKA and CKI in vitro and that blocking either activity in cells results in decreased levels of phosphorylated Smo protein. Thus, Smo is a direct target of PKA and CKI, but is this relevant to Hh signaling during development? In both studies Smo phosphorylation site mutants failed to fully rescue smo mutant phenotypes while phosphomimetic mutants resulted in constitutive Smo activity and ectopic expression of Hh target genes. Furthermore, the levels of Smo activity appeared to correlate with the levels of its phosphorylation.
While it is unclear how phosphorylation by PKA and CKI affects Smo activity, both groups concluded that it regulates Smo stability on the cell surface. Apionishev et al. observed that Smo possessing mutated PKA and CKI sites is resistant to the Ptc-dependent proteolysis that normally occurs in anterior cells, whereas Jia et al. found that phosphorylated Smo accumulates on the cell surface more readily than unphosphorylated Smo. Moreover, Hh signaling appears to promote Smo accumulation in response to phosphorylation, but whether it does so by regulating phosphorylation by PKA and CKI is still subject to further investigation. Smo hyperphosphorylation is essential for its accumulation and high-threshold Hh signaling. Hh, however, also controls Smo cell surface accumulation and pathway activation by mechanisms independent of PKA phosphorylation, thus rendering its relationship with Smo even more intricate.
Myrto Raftopoulou Signaling Gateway
References
Jia, Jianhang, Tong, Chao, Wang, Bing, Luo, Liping, & Jiang, Jin. Hedgehog signalling activity of Smoothened requires phosphorylation by protein kinase A and casein kinase I. Nature432, 1045–1050 (23 December 2004); nature03179 | Article | PubMed |
Apionishev, Sergey , Katanayeva, Natalya M., Marks, Steven A., Kalderon , Daniel, & Tomlinson, Andrew. Drosophila Smoothened phosphorylation sites essential for Hedgehog signal transduction. Nature Cell Biology7, 86–92 (12 December 2004); ncb1210 | Article |
