Hh Signal Transduction | Formation of Active Hh Protein
There are four main processing events: signal sequence cleavage, autoproteolytic-cleavage generating HhNp (Lee et. al., 1994), cholesterol addition (Porter et. al.,1996a) and palmitic acid addition by an acyltransferase (Lee et. al.,2001; Chamoun et. al., 2001).
The C-terminal half of Hh shows homlogy to HINT domain containing proteins involved in intramolecular cleavage. Key residues involved in processing are gly, cys and phe (Lee et. al., 1994). The thiol on the cys begins a nucleophilic attack on the carbonyl group of gly leading to displacement of the amine. This leads to a thioester bond between the carbonyl group of gly and the cys side chain and a sulfur to nitrogen rearrangement (Porter et. al.,1996a; Porter et. al.,1996b).
A second nucleophile attached to cholesterol then attacks the carbonyl oxygen on gly, which then shifts the electrons to the Sulfur group, leading to cleavage and covalent attachment of cholesterol to the N-terminal portion of Hh (HhNp) (Porter et. al.,1996a).
The cleavage results in cholesterol bound to the N part of Hh at its new C- terminus (HhNp) (Porter et. al.,1996a). HhNp is the signaling portion of the Hh protein..
Palmitic acid is attached to HhNp at the amino terminal Cys marked by a conserved motif (Cys, Gly, Phe, Gly and Arg) (Pepinsky et. al.,1998). It is unknown how palmitic acid is attached to the N-terminus of HhNP, but it is thought to be aided by an acyltransferase encoded by the gene rasp (also known as sightless and skinny hedgehog (Lee et. al.,2001; Chamoun et. al., 2001) Palmitoylation is required for full activation of Hedgehog signaling targets (Lee, J. D. and Treisman, J. E.,2001)..
