Introduction

Protein phosphorylation plays a cardinal role in regulating many cellular processes in eukaryotes. In particular, protein phosphorylation is a major currency of signal transduction pathways. Processes that are reversibly controlled by protein phosphorylation require not only a protein kinase but also a protein phosphatase. Target proteins are phosphorylated at specific sites by one or more protein kinases and these phosphates are removed by specific protein phosphatases. In principle the extent of phosphorylation at a particular site can be regulated by changing the activity of the cognate protein kinase or protein phosphatase or both.

Protein tyrosine kinases (PTKs) catalyse the transfer of γ- phosphate of ATP to tyrosine residues of protein substrates and are critical components of signalling pathways that control a variety of biological responses including cell proliferation, migration, differentiation and survival. PTKs can be subvided in two large families: receptor tyrosine kinases (RTKs) and non-receptor tyrosine kinases (NRTKs). Although platelets contain a small number of receptor tyrosine kinases on their surface, including the PDGF receptor, the majority of tyrosine kinases identified in platelets thus far are non-receptor forms, including members of the Src-kinase family, Syk kinase family, focal adhesion kinase (FAK) family, the Janus kinase (JAK) family and the Tec kinase family. Because of the key roles PTKs play in cellular signalling processes, their catalytic activity is tightly controlled in normal cells by protein tyrosine phosphatases, by other protein tyrosine or serine/threonine kinases and by autoregulatory mechanisms.