A PKA Inhibitor Motif within Smoothened Controls Hedgehog Signal Transduction

Abstract

The Hedgehog (Hh) cascade is central to development, tissue homeostasis, and cancer. A pivotal step in Hh signal transduction is the activation of GLI transcription factors by the atypical G protein-coupled receptor (GPCR) Smoothened (SMO). How SMO activates GLI has remained unclear for decades. Here we show that SMO employs a decoy substrate sequence to physically block the active site of the PKA catalytic subunit (PKA-C) and extinguish its enzymatic activity. As a result, GLI is released from phosphorylation-induced inhibition. Using a combination of in vitro, cellular, and organismal models, we demonstrate that interfering with SMO / PKA pseudosubstrate interactions prevents Hh signal transduction. The mechanism we uncovered echoes one utilized by the Wnt cascade, revealing an unexpected similarity in how these two essential developmental and cancer pathways signal intracellularly. More broadly, our findings define a new mode of GPCR-PKA communication that may be harnessed by a range of membrane receptors and kinases.

Regulation of Smoothened Trafficking and Abundance in Hedgehog Signaling

“…It is interesting to note that Hh stimulates the binding of PKA catalytic subunit (PKAc) to SmoCT in both Drosophila and mammalian systems ( Li et al, 2014 ; Ranieri et al, 2014 ; Arveseth et al, 2021 ). While binding of PKAc to dSmo is thought to promote dSmo phosphorylation and sequester PKAc away Ci, binding of PKAc to a PKI motif in vertebrate SmoCT has been shown to sequester PKAc away from Gli2 and Gli3 and thus inhibit Gli phosphorylation and processing ( Happ et al, 2021 ). It would be interesting to determine whether a similar PKI motif is present in dSmo and contributes its inhibition of Ci phosphorylation.…”