Activation of SMO

PTCH-mediated inhibition of SMO is relieved upon ligand stimulation of PTCH, but the mechanisms for this relief are again unknown. SMO and PTCH appear to have opposing localizations in both the 'off' and 'on' state, with PTCH exiting and SMO entering the cilium upon Hh pathway activation (Denef et al, 2000; Rohatgi et al, 2007; reviewed in Goetz and Anderson, 2010; Hui and Angers, 2011). Activation of SMO involves a conserved phosphorylation-mediated conformational change in the C-terminal tails that destabilizes an intramolecular interaction and promotes the interaction between adjacent tails in the SMO dimer. In Drosophila, this phosphorylation is mediated by PKA and CK1, while in vertebrates it appears to involve ADRBK1/GRK2 and CSNK1A1. Sequential phosphorylations along multiple serine and threonine motifs in the SMO C-terminal tail appear to allow a graded response to Hh ligand concentration in both flies and vertebrates. In flies, Smo C-terminal tail phosphorylation promotes an association with the Hedgehog signaling complex (HSC) through interaction with the scaffolding kinesin-2 like protein Cos2, activating the Fu kinase and ultimately releasing uncleaved Ci from the complex (Zhang et al, 2005; Ogden et al, 2003; Lum et al, 2003; reviewed in Mukhopadhyay and Rohatgi, 2014). In vertebrates, SMO C-terminal tail phosphorylation and conformational change is linked to its KIF7-dependent ciliary accumulation. In the cilium, SMO is restricted to a transition-zone proximal region known as the EvC zone (Yang et al, 2012; Blair et al, 2011; Pusapati et al, 2014; reviewed in Eggenschwiler 2012). Both SMO phosphorylation and its ciliary localization are required to promote the Hh-dependent dissociation of the GLI:SUFU complex, ultimately allowing full-length GLI transcription factors to translocate to the nucleus to activate Hh-responsive genes (reviewed in Briscoe and Therond, 2013).