Src homology 2 (SH2) domains are small (around 100 amino-acid residues) independently folding protein modules that bind to short phosphotyrosine-containing peptides found in activated target molecules including several growth-factor receptors. SH2 domain containing proteins within the cell are crucial for mediating the mediating the hormone-activated receptor signal further downstream in the cell. Phosphoinositide 3-kinase (PI3K) is one such SH2-domain containing protein.
The first part of the signalling pathway can be visualized as follows: First, the growth-factor binds to the gowth-factor receptor. This could be platelet-derived growth factor (PDGF) binding to the platelet-derived growth factor receptor (PDGF-R). The binding of PDGF to the receptor induces a dimerization of the receptor molecules which subsequently causes autophosphorylation of specific tyosines on the intra-cellular region of the receptor. In the case of PDGF-R, the tyrosines 740 and 751 are phosphorylated in this dimerization process. The two SH2 domains of PI3K has a high affinity for these two phosphorylated tyrosines. The next step in the signalling pathway involves the binding of PI3K which is an inactive cytoplasmic kinase when not bound. Binding of PI3K to PDGF-R activates the kinase activity of PI3K, and PI3K now begins to phosphorylate phosphoinositides in the cell membrane in the 3-position and lipases cut off the fatty-acid chain linking the phosphoinositol ring to the membrane. The released phosphoinositols are small soluble molecules which diffuse to the interior of the cell where they act as downstream signalling molecules (second messengers) and eventually cause the cell to respond to the primary hormone signal.
PI3K is made up of a regulatory (p85) and a catalytic (p110) subunit. The regulatory p85 subunit has amongst others, the two SH2 domains which are responsible for binding to the activated PDGF-R molecule. The p110 subunit has the kinase activity which is activated when the two SH2 domains of p85 binds to the PDGF-R molecule. One of our long-term goals are to understand how the binding of the phosphopeptide to the two SH2 domains in the regulatory p85 subunit is transmitted to the catalytic p110 subunit and in order to pursue this, we are investigating one of the the two SH2 domains, namely the C-terminal SH2 domain of the regulatory p85 subunit of PI3K.