<p>The hedgehog proteins are composed by two main domains: the Hedge (N-terminal) and Hog (C-terminal) domains. The Hedge domain forms the HhN portion of the Hh proteins (together with the signaling sequence, SS) that is separated from the rest of protein by an auto-cleavage reaction preformed by the Hog domain <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074132#pone.0074132-Lee1" target="_blank">[4]</a>. The Hog domain forms the HhC portion of the Hh proteins and shares similarity with self-splicing Inteins on the Hint module <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074132#pone.0074132-Burglin2" target="_blank">[22]</a>. The auto-cleavage reaction occurs on a GCF (glycine-cysteine-phenylalanine) motif that forms the boundary between the two main parts of the Hh proteins, with the cysteine residue initiating a nucleophilic attack on the carbonyl carbon of the preceding residue, the glycine <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074132#pone.0074132-Beachy1" target="_blank">[5]</a>. The sterol-recognition region (SRR) forms the C-terminal region of the Hog domain and binds a cholesterol moiety that acts as an electron donor on a second nucleophilic attack that results in the cleavage of the bound between the glycine and cystein residues and in the attachment of the cholesterol moiety to the glycine residue <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074132#pone.0074132-Beachy1" target="_blank">[5]</a>. After auto-cleavage, the sterified HhN fragment is further palmitoylated on the cystein residue immediately after the SS region <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074132#pone.0074132-Chamoun1" target="_blank">[13]</a> and leaves the endoplasmic reticulum (ER) for later export <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0074132#pone.0074132-Chen3" target="_blank">[11]</a>.</p

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