The SH2 domain.

One of the first oncogenes discovered was a protein called Src. This was found in the Rous sarcoma virus and was shown to be the causative agent of chicken tumors. It was discovered in 1911 by by Peyton Rous, and was early evidence that cancer could, at least sometimes, be caused by a virus. In the 1970 and 80’s Michael Bishop and Harold Varmus cloned and sequence the Rous sarcoma genome and discovered one gene, which they called Src, was necessary and sufficient for transformation, i.e. tumor formation. Interestingly the viral Src, or v-Src, was almost but not quite identical to a chicken gene called c-Src, or cellular Src. The virus had somehow incorporated a variant of the normal gene which gave it a selective advantage by producing tumors. People obviously looked at the sequence of src very carefully when it became available. They found three regions of homology with other sequences, which they called src-homology domains. For some reason they numbered them from the C-terminus, calling them src-homology 1, 2 and 3, (SH1, SH2 and SH3). The protein also has an N-terminal myrisotylation consensus sequence which allows lipid modification which anchors the protein to the membrane. The SH1 domain is about 280 amino acids and was homologous to sequences found in all protein kinases, and was quickly shown to represent a protein kinase domain. The sequence was a little different from known protein kinases, which were all Ser/Thr kinases, and src was in fact one of the first tyrosine kinase isolated. SH2 and SH3 domains were more difficult to figure but were also more interesting. Both were found in a wide variety of molecules many of which were involved in signal transduction. So, phospholipase C-g has both SH2 and SH3 domains, as do PI-3 kinase, as do several regulators of ras (the ras-GRF SOS, the p120 ras-GAP NF1, see below) and other small G-proteins. Many adapter molecules (e.g. Shc, Grb-2) also have these domains.

The domains can also be found alone (e.g. SH2 in tensin, a protein found at adhesion sites in fibroblasts, SH3 domains in some myosin I types, a -spectrin, PSD-95). Frequently proteins have two SH2 domains (e.g. PLC-g , p85 subunit of PI3 kinase, Syk and Zap kinases, SHIP and other tyrosine phosphatases) or two SH3 domains (e.g. Grb-2, vav). Since many of these molecules are known to have roles in signal transduction it was widely assumed that the SH2 and SH3 domains would have some important function.

Many of the proteins with SH2 domains bind to tyrosine phosphorylated growth factor receptors and are localized under the plasma membrane where they are able to act on their substrates, which can be membrane lipid (PLC-g , PI-3 kinase), membrane proteins (GAPs and GRFs for ras, rac/rho etc.). A major signaling pathway involves the small G proteins of the Ras family. Some more history.

The SH2 is a compact module of about 100 amino acids with the N and C terminus very close together. There are some absolutely conserved residues, which we now know are responsible for bind to tyrosine phosphates. These are generally at the N-terminus of the module. The SH2 domains are more variable at the C-terminus, and examination of the b D strand shows how you can figure out the specificity of the SH2 domain. The binding of an SH2 to the correct ligand is 10-100nm, comparable to an antibody antigen interaction. The binding is absolutely dependent on the phosphotyrosine residue, and high affinity further requires the correct peptide following the phosphotyrosine.