Three Hedgehog proteins are found in mammals; these are sonic hedgehog (gene SHH), Indian hedgehog (gene IHH) and desert hedgehog (gene DHH). They are first synthesised as a 45 kDa precursor, which undergoes an autocatalytic cleavage releasing a 19 kDa N-terminal peptide involved in signalling and a 45 kDa C-terminal peptide, which is involved in cleavage and bears a catalytic activity of cholesterol transferase.

HHG proteins are subjected to post-translational modifications that are required for their activity; on the C-terminal side, they are covalently bound to a cholesterol molecule; on the N-terminal side, a specific palmitoyltransferase, HHAT (hedgehog acyltransferase) or SKI (skinny hedgehog), binds them to a palmitic acid moiety at the level of a cysteine residue. When equipped with these two hydrophobic ligands, HHG proteins can be inserted in the peripheral phospholipid layer of a lipoprotein and thus form a multimeric complex (Fig. 9.1).

The HHG ligands with their two lipid substituents are recognised on a target cell by a receptor called patched (PTCH). There are two distinct PTCH proteins, encoded by the genes PTCH1 and PTCH2, which seem to have different functions, the first one being the best known and the only one to be considered here. PTCH belongs to the same transporter family as DISP and also presents 12 transmembrane domains and a cholesterol-binding domain. It also presents two large extracellular domains, which ensure HHG binding. PTCH is only an intermediary in HHG signal reception, since it transfers the information thus received to another membrane protein called smoothened (gene SMO), by releasing the constitutive inhibition exerted in this protein. The mechanism of this release is not intimately known, but it seems that no direct interaction exists between PTCH and SMO. PTCH would only be a transport protein and would control the localisation of a small inhibitory or excitatory molecule that remains to be identified. In the absence of signal, PTCH would pump an SMO inhibitory molecule from the inside to the outside of the cell; when bound to HHG, PTCH would cease pumping and SMO would then be functional; the reverse would be true if the molecule could conversely activate SMO (Fig. 9.2). Some authors hypothesise that the small molecule transported by PTCH and interacting with SMO could be vitamin D3 or an oxysterol.

SMO is a membrane protein with seven transmembrane domains, belonging to the fifth family of GPCRs (G-protein-coupled receptors) (Chap.​ 6) as FZD receptors (Chap.​ 7). When PTCH binds an HHG ligand, SMO accumulates in primary cilia, which are subcellular organelles present in all vertebrate cells and form protrusions at the surface of the plasma membrane, devoid of motility and based upon a microtubule cytoskeleton. SMO migration to primary cilia is required for Hedgehog signalling. This migration is accompanied by SMO phosphorylation by GRK2 (GPCR kinase 2, gene ADRBK1), which is thought to be required for migration. Proteins other than PTCH are able to bind HHG ligands and modulate the signal brought to the target cell: CDO (cell adhesion molecule-related/downregulated by oncogenes) and BOC (brother of CDO), which are members of the immunoglobulin superfamily and function as accessory receptors to HHG; it is not clear whether or not they are required for signal transmission. They operate via a glycosaminoglycan (heparan sulphate)-binding site. Another protein, HHIP (hedgehog-interacting protein), would be a decoy receptor for HHG ligands and would thus inhibit this signalling pathway.