3.5 Summary of Section 3
Protein domains allow segregation of different functions in the same protein. They can have a binding function, a structural function or a catalytic function.
Binding domains mediate interactions between proteins of related function (such as those in a signalling cascade) and often are important in regulation of activity. Interactions via these binding domains are often dependent on the phosphorylation state of one of the binding partners. Examples of binding domains in signalling proteins include SH2 and SH3 domains.
The tyrosine kinase Src contains an SH2 and an SH3 domain. These domains regulate the kinase activity of Src, which is associated with two further domains of the protein. Activation of Src kinase activity requires removal of an inhibitory phosphate group from a tyrosine residue near the C-terminus and phosphorylation of another tyrosine residue in the activation loop. These events can occur only if intramolecular interactions are disrupted by binding of the SH2 and SH3 domains to other specific ligands.