Sonic Hedgehog: Difference between revisions

'''SONIC HEDGEHOG'''

{{STRUCTURE_1vhh | PDB=1vhh  |  SCENE=Sandbox_191/Scenedefault/4}}

Sonic hedgehog (Shh) is a member of the Hedgehog (Hh) family of secreted extracellular signaling proteins, which serve important roles in regulating both short-range and long-range patterning processes in developing invertebrate and vertebrate tissues<ref>PMID: 7867057</ref>. First discovered in ''Drosophila'', where mutations of the single ''Hedgehog'' gene produces larvae that are covered in hedgehog-like denticles, Hh proteins are encoded by at least three genes in mammals - ''Sonic'', ''Desert'', and ''Indian hedgehog''<ref>PMID: 7916661</ref>. With the ability to control such fundamental processes as pattern formation in vertebrate limb buds<ref>PMID: 8269518</ref>, the formation of motor neurons in the neural tube <ref>PMID: 7736596</ref>, and the development and maintenance of tissues and organs<ref>PMID: 10980429</ref>, Shh is the most well-studied member of the Hh signaling proteins<ref>PMID: 10753901</ref>. Excessive signaling in adult cells has been implicated in the development of several human cancers<ref>PMID: 14737121</ref><ref name="Path">PMID: 12044012</ref>.  

As with all members of the Hh family, Shh biosynthesis begins with an unusual molecular processing event. Following cleavage of its signal peptide, the Shh precursor protein is autocatalytically cleaved into two functionally distinct domains, a 19-kDa amino-terminal domain (Shh-N) and a 27-kDa carboxy-terminal domain (Shh-C)<ref>PMID: 7891723</ref>. Spanning residues 24 to 197 in human Shh, Shh-N is responsible for all of the local and long-range signaling activities of Shh. Shh-C possesses an intramolecular transferase activity responsible for covalent attachment of a molecule of cholesterol to the C-terminus of Shh-N. The addition of cholesterol serves to tether Shh-N to the cell membrane, restricting its range of activity to that of local signaling only<ref>PMID: 8824192</ref>. A second modification involving the attachment of a palmitoyl group to Cys-24 on the protein's N-terminus has recently been discovered in insect and mammalian cells. This N-terminal modification is thought to increase the potency of the Shh-N signal as much as 30-fold<ref>PMID: 9593755</ref>.     

The three-dimensional structure of murine Shh-N (residues 39-195) is shown as 1VHH. An α + β sandwich consisting of two <scene name='Sandbox_191/Scene2/5'> α-helices</scene> and a six-stranded, mixed <scene name='Sandbox_191/Scene3/5'> β-sheet</scene> makes up the core of the structure, along with a two-stranded, antiparallel β-sheet<ref name="Palm"/>. Although this type of folding arrangement has not yet been seen in other proteins, the presence of a <scene name='Sandbox_191/Scene4/3'>tetrahedrally coordinated zinc ion</scene> in Shh-N bears close structural resemblance to the zinc coordination sites of zinc hydrolases, including thermolysin and carboxypeptidase A. Three amino acid side chains – <scene name='Sandbox_191/Scene4/4'>His 141, Asp 148, and His 183</scene> – are bound to the zinc ion in the crystal structure, along with a single <scene name='Sandbox_191/Scene4/5'>molecule of water</scene>. [[Image:Catalytic site.png |left| thumb | '''Figure 1.''' A close-up of the zinc coordination site of Shh-N, showing His 141, Asp 148, and His 183 separated by distances of 2.06, 1.97, and 2.08 Å, respectively. The zinc-bound water molecule is also shown in line with Glu 177, which is thought to participate in hydrolysis by abstracting a proton from the water molecule<ref name="Palm"/>.]] Zinc ions that serve a structural role in proteins are normally coordinated by four amino acid side chains and are not usually exposed to the surrounding solvent. The presence of a zinc-bound water molecule in Shh-N, by contrast, is indicative of a catalytic function. In zinc hydrolases, the water molecule is key to the protein's enzymatic activity when its proton is removed by a nearby glutamate residue. <scene name='Sandbox_191/Scene4/6'>Glu 177</scene> (Figure 1) likely serves the same role in Shh-N, further supporting a novel, hydrolytic function for this protein. Based on the catalytic mechanisms for thermolysin and carboxypeptidase A, three non-coordinating residues in Shh-N (<scene name='Sandbox_191/Scene4/7'>His 135, His 181, and Glu 127</scene>) are also believed to participate in a potential hydrolysis reaction<ref name="Palm">PMID: 7477329</ref>.  

The crystal structure of Shh-N contains a single sulphate molecule.  

The tetrahedrally coordinated zinc ion of Shh-N, along with the non-coordinating residues thought to assist hydrolysis, are highly conserved among vertebrate Hh proteins. A potential hydrolytic activity is therefore expected to play an important cellular role. In pursuit of a substrate for Shh-N, it was found that <scene name='Sandbox_191/Scene3/6'>Ala 194 and Lys 195</scene> near the C-terminus of one Shh-N molecule can hydrogen bond with residues in the zinc binding site of a second Shh-N molecule. This indicates that the protein may be capable of cleaving between Lys 195 and Ser 196 within its own C-terminus <ref name="Palm"/>. This is the most highly conserved region of Hh proteins<ref>PMID: 8807822</ref>. The suspected hydrolytic function of Shh-N has been suggested to liberate the tethered protein from the cell membrane to facilitate long-range signaling <ref name="Palm"/>. However, other possible substrates for Shh-N proteolysis are also likely, including an Shh receptor or other signaling proteins involved in the Shh pathway.