Meta-Biol

• Pathways

Autoproteolytic processing of the Hh precursor is essential for the production of active secreted Hh ligand and mutants that disrupt this processing have been identified in the congenital nervous system disorder holoprosencephaly (Traiffort et al, 2004; Maity et al, 2005; Roessler et al, 2009; reviewed in Jiang et al, 2008). Cleavage of Hh occurs through two nucleophilic substitutions. The first step is mediated by the catalytic cysteine residue, which is found in a conserved G-C-F motif. The cysteine side chain attacks the carbonyl carbon of the main peptide chain between the glycine and cysteine residues, replacing the amino group in the peptide backbone with a thioester linkage (Lee et al, 1994; Porter et al, 1995; Porter et al, 1996a, b; Chen et al, 2011). The second step involves nucleophilic attack of the same carbonyl group by cholesterol. This step displaces the C-terminal fragment (Hh-C) of the Hh precursor and results in the formation of the N-terminal fragment (Hh-Np) modified at its C-terminus by an ester linkage with cholesterol (Porter et al, 1996a, b; Chen et al, 2011). Cholesterol modification appears to contribute to further processing and trafficking of the Hh ligand, as engineered forms of vertebrate and fly Hh that lack cholesterol are not efficiently palmitoylated (Pepinsky et al, 1998). Cholesterol also restricts the diffusion of the secreted ligand by interacting with the lipid bilayer of the secreting cell. Consistent with this, aberrant activation of Hh target genes is seen in the absence of cholesterol modification (Peters et al, 2004; Guerrero et al, 2007; Li et al, 2006; Huang et al, 2007).