Unconventional GPCR-PKA Signaling in the Hedgehog Pathway.

The Hedgehog pathway controls tissue and organ development throughout animal evolution and drives several widespread skin and brain cancers. 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. The principal issue is that SMO appears to defy standard signaling paradigms utilized by nearly all other GPCRs. 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. Our findings help to resolve a longstanding mystery in developmental and cancer signaling, and reveal a new mechanism of receptor-kinase communication that may apply broadly throughout the GPCR superfamily. Our work also suggests new mechanism-inspired therapeutic strategies to manage cancers driven by ectopic Hedgehog pathway activity.