Repression of Hh signaling in the absence of ligand depends on the transmembrane receptor protein Patched (PTCH), which inhibits Smoothened (SMO) activity by an unknown mechanism. This promotes the proteolytic processing and/or degradation of the GLI family of transcription factors and maintains the pathway in a transcriptionally repressed state (reviewed in Briscoe and Therond, 2013). In the absence of ligand, PTCH is localized in the cilium, while SMO is largely concentrated in intracellular compartments. Upon binding of Hh to the PTCH receptor, PTCH is endocytosed, relieving SMO inhibition and allowing it to accumulate in the primary cilium (Marigo et al, 1996; Chen and Struhl, 1996; Stone et al, 1996; Rohatgi et al, 2007; Corbit et al, 2005; reviewed in Goetz and Anderson, 2010). In the cilium, SMO is activated by an unknown mechanism, allowing the full length transcriptional activator forms of the GLI proteins to accumulate and translocate to the nucleus, where they bind to the promoters of Hh-responsive genes (reviewed in Briscoe and Therond, 2013).
In addition to PTCH, three additional membrane proteins have been shown to bind Hh and to be required for Hh-dependent signaling in vertebrates: CDON (CAM-related/downregulated by oncogenes), BOC (brother of CDO) and GAS1 (growth arrest specific 1) (Yao et al, 2006; Okada et al, 2006; Tenzen et al, 2006; McLellan et al, 2008; reviewed in Kang et al, 2007; Beachy et al, 2010; Sanchez-Arrones et al, 2012). CDON and BOC, homologues of Drosophila Ihog and Boi respectively, are evolutionarily conserved transmembrane glycoproteins that have been shown to bind both to Hh ligand and to the canonical receptor PTCH to promote Hh signaling (Okada et al, 2006; Yao et al, 2006; Tenzen et al, 2006, McLellan et al, 2008; Izzi et al, 2011; reviewed in Sanchez-Arrones et al, 2012). Despite the evolutionary conservation, the mode of ligand binding by CDON/Ihog and BOC/Boi is distinct in vertebrates and invertebrates. High affinity ligand-binding by CDON and BOC requires Ca2+, while invertebrate ligand-binding is heparin-dependent (Okada et al, 2006; Tenzen et al, 2006; McLellan et al, 2008; Yao et al, 2006; Kavran et al, 2010). GAS1 is a vertebrate-specific GPI-anchored protein that similarly binds both to Hh ligand and to the PTCH receptor to promote Hh signaling (Martinelli and Fan, 2007; Izzi et al, 2011; reviewed in Kang et al, 2007). CDON, BOC and GAS1 have partially overlapping but not totally redundant roles, and knock-out of all three is required to abrogate Hh signaling in mice (Allen et al, 2011; Izzi et al, 2011; reviewed in Briscoe and Therond, 2013).