br Conclusions br Acknowledgements br Protein tyrosine kinas
Protein tyrosine kinases (PTKs) of Src family are important components in cellular signal transduction pathways that couple diverse extracellular signals to appropriate cellular responses . Their activities are tightly regulated in the cells and aberrant activation of some of these PTKs is associated with the development of many types of human cancer . One key mechanism for their regulation is the phosphorylation of a Tyr residue near their N-termini (Tyr527 in Src), which results in their inactivation . Two PTKs, -terminal rc inase (Csk) and sk omologous inase (Chk) , have been identified as responsible for the phosphorylation of Tyr527. Chk and Csk share a 53% identity and 70% homology in amino Mubritinib sequence , . Although this sequence homology is not significantly higher than with some other protein tyrosine kinases, several key structural features indicate that Chk and Csk belong to one family. First, the two PTKs share an overall structural organization of SH3, SH2, and the SH1 catalytic domain in the order of N- to C-terminus. Second, both enzymes lack the SH4 myristoylation signal, the autophosphorylation site, and the regulatory phosphorylation sites that are common to the closest PTK family, Src kinases . Third, Csk in a variety of experimental settings has been demonstrated to regulate Src PTKs , and mouse Chk gene product has also been shown to phosphorylate Lck, a member in the Src family , on the C-terminal regulatory Tyr residue, demonstrating their functional similarity. These observations suggested that Chk and Csk belong to the same PTK family. Despite the extensive similarities, recent studies have noted substantial differences between the two enzymes. First, while Csk is ubiquitously expressed , Chk is expressed only in the brain, NK cells, and activated T cells , . Second, Csk is highly expressed in normal breast cells and breast cancer cells, but appears unable to suppress the activated Src found in breast cancer cells. On the other hand, Chk is not expressed in normal breast cells, becomes induced in breast cancer cells, and is able to suppress Src activity , . The ability of Chk to suppress Src activity is correlated to its ability to bind to phosphorylated Tyr1248 of ErbB2 receptor kinase through its SH2 domain . Chk also binds to c-Kit receptor tyrosine kinase and regulates the Fyn and Lyn kinases, two members of the Src family , . Chk but not Csk is found to suppress the Lyn kinase activity during platelet activation . These studies indicate that Chk and Csk have distinct expression patterns, mechanisms of regulation, and mode of action.
Introduction Csk (C-terminal Src kinase) is a ubiquitously expressed 50 kDa protein tyrosine kinase, the main function of which is to downregulate the catalytic activity of Src-family kinases , , . Together with CHK (Csk homologous kinase)  it belongs to the Csk-family of non-receptor tyrosine kinases. Csk is a modular protein consisting of SH3-, SH2- and kinase domains. However, Csk is unique among the tyrosine kinases because it lacks a conserved tyrosine autophosphorylation site in the kinase domain. Furthermore, Csk does not possess a myristylation site in the N-terminus or a regulatory phosphotyrosine site in the C-terminal tail. Several studies have shown that the catalytic activity of Csk can be modulated to some extent , , . The crystal structure of four active and two inactive full-length Csk molecules provides the molecular basis to study how control of Csk activity is achieved . The crystal data show that in all Csk molecules, the SH3 and SH2 domains align on top of, but on opposite sides of the N-terminal lobe of the kinase domain. Furthermore, both the SH3–SH2– and the SH2–kinase linkers make extensive interactions with the kinase domain (Fig. 1A). The crystal data also show that in inactive Csk molecules, the SH2 domains are rotated upward, resulting in loss of some of the contacts between the SH2- and the kinase domain.