An adaptor protein is a protein which is accessory to main proteins in a signal transduction pathway. These proteins tend to lack any intrinsic enzymatic activity themselves but instead mediate specific protein-protein interactions that drive the formation of protein complexes. Examples of adaptor proteins are Shc and Grb2. Much of the specificity of signal transduction depends on the recruitment of several signalling components such as protein kinases and G-protein GTPases into short lived active complexes in response to an activating signal such as a growth factor binding to its Receptor_(biochemistry). Adaptor proteins usually contain several domains within their structure (e.g., Src homology 2 (SH2) and SH3 domains) which allow specific interactions with several other specific proteins. SH2 domains recognise specific amino acid sequences within proteins containing phosphotyrosine residues and SH3 domains recognise proline-rich sequences within specific peptide sequence contexts of proteins. There are many other types of interaction domains found within adaptor and other signalling proteins which allow a rich diversity of specific and coordinated protein-protein interactions to occur within the cell during signal transduction.