Sonic Hedgehog

Sonic HedgehogSonic Hedgehog

IntroductionIntroduction

Sonic hedgehog (Shh) is a member of the Hedgehog (Hh) family of secreted extracellular signaling proteins that serve important roles in regulating both short-range and long-range patterning processes in developing invertebrate and vertebrate tissues . First discovered in Drosophila, where mutations of the 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 . With the ability to control such fundamental processes as pattern formation in vertebrate limb buds, the formation of motor neurons in the neural tube, and the development and maintenance of tissues and organs, Shh is the most well-studied member of the Hh signaling pathway. Excessive signaling in adult cells has been implicated in the development of several human cancers .

BiosynthesisBiosynthesis

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 a 19-kDa amino-terminal domain (Shh-N) and a 27-kDa C-terminal domain (Shh-C) . 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. Addition of cholesterol serves to tether Shh-N to the cell membrane, restricting its range of activity to that of local signaling only . Thus, the two domains of Shh are catalytically distinct.

Structural OverviewStructural Overview

The proposed structure for the native Shh-N protein (1VHH) from Cys 25 to Gly 198 is shown. An α + β sandwich consisting of two and a six-stranded, mixed makes up the core of the structure, along with a two-stranded, antiparallel β-sheet. Although this type of folding arrangement has not yet been seen in other proteins, the presence of a 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 – – are bound to the zinc ion in the crystal structure, along with a single . Zinc ions that serve a structural role in proteins are normally coordinated by four amino acid side chains, including a cysteine, 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, a zinc-bound water molecule is key to the protein's enzymatic activity when its proton is removed by a nearby glutamate residue. is believed to serve an analogous role in Shh-N, further supporting a novel, hydrolytic function for this protein. Based on the catalytic mechanisms for thermolysin and carboxypeptidase A, other residues in Shh-N that are believed to be involved in a potential hydrolysis mechanism include .

The tetrahedrally coordinated zinc ion of Shh-N, along with the non-coordinating residues believed to assist in 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 Ala 194 and Lys 195 near the C-terminus of one Shh-N molecule can hydrogen bond with residues in the zinc binding site of a second molecule of Shh-N. This indicates that the protein may be capable of cleaving its own C-terminus, a function that has been suggested to liberate the tethered Shh-N from the cell membrane to facilitate long-range signaling.

ReferencesReferences

Please do NOT make changes to this Sandbox until after April 23, 2010. Sandboxes 151-200 are reserved until then for use by the Chemistry 307 class at UNBC taught by Prof. Andrea Gorrell.