Cell Signalling
Both receptor and non-receptor protein tyrosine kinases are essential enzymes in cellular signalling processes regulating cell growth, differentiation, migration and metabolism. Considerable evidence implicates tyrosine kinases in the development of many types of cancer and leukaemia via their involvement in numerous growth factor signalling cascades. Members of the Src family of tyrosine kinases are signalling intermediates that can control aspects of these processes. Some of these molecules were originally identified as viral oncogenes and are potent carcinogens in animal models, although comparable genetic alterations in human cancers are rarely observed. However, the level and/or activity of these kinases are often elevated in human tumours and their activity can correlate with metastatic potential. Recent advances in the development of small molecule inhibitors of tyrosine kinases has resulted in great success in treating particular neoplasms in cellular and animal models. The therapeutic advantage of these reagents is illustrated by the enormous success of Imatinib mesylate (Gleevec, STI571) and related compounds for the treatment of chronic myeloid leukaemia (CML).
Senior Research Staff
Dr Evan Ingley
Head, Cell Signalling
Research: Lyn signalling complexes and pathways; cell signalling |
|
Research Details
The Lyn tyrosine kinase
![[caption below]](/images/misc/Src-Family-lge.gif)
General structural schematic of the Src family kinases in their inactive and active configurations. Left, the inactive configuration showing the SH2 domain interacting with the phosphorylated C-terminal tyrosine (pY508), the SH3 domain interacting with the SH2-kinase connector which forms a left-handed polyproline type II helix, and the dephosphorylated activation loop (Y397) folded back over the substrate binding site. Right, the active configuration, showing SH2 and SH3 domains released from the intramolecular interactions and available for binding to substrates and regulatory molecules, the C-terminal tyrosine is dephosphorylated (Y508), and the activation loop is phosphorylated (pY397) and is folded away from the substrate binding site and allows the two kinase lobs (N and C) to form a kinase competent catalytic cleft.
Lyn is a member of the Src family of intracellular membrane-associated tyrosine kinases. While the amino terminus of each member is unique, this family shares significant homology in the kinase domain, as well as the SH2/SH3 protein interaction domains. Lyn is involved in transmitting signals from several cytokine receptors for haemopoietic cells of erythroid/myeloid and B lymphoid origin, neuronal cells, prostate cells and colon cells through phosphorylating a wide range of important substrates, consequently affecting the function of these molecules.
Lyn signalling and regulation
Our studies have shown that Lyn plays a crucial role in the Epo-induced terminal differentiation of erythroid cells. These were the first demonstrations of a Src kinase family member playing a major role in Epo signalling, and not just involving the JAK2 kinase. Our studies support the model of Epo-receptor signalling where JAK2 phosphorylates the receptor (as the primary kinase), creating docking sites for STAT5, which is then phosphorylated and activated by Lyn (as the secondary kinase). We have extended our original observations on the role of Lyn in erythroid differentiation by attempting to unravel the signalling cascades activated by this kinase. Using a yeast two-hybrid screen, we have identified a number of fascinating molecules that interacted with Lyn. One interacting molecule we identified, Csk binding protein (Cbp), is important in controlling the activity status and protein levels of Lyn and other Src family kinases (SFK) as well as co-ordinating SFK regulation of other signalling pathways (eg PI3 kinase/Akt and PLCγ). Others, such as Lyn Associated Cytoskeletal Modulator (LACM), link Lyn activation with the cytoskeleton and cell membrane, mediating substantive cell morphological changes that affect cell proliferation, attachment and motility.
As with other Src family kinases Lyn is regulated by protein interactions through its SH2/SH3 domains and phosphorylation status (see figure). In its inactive state Lyn is phosphorylated at its carboxyl terminus on Y508 creating a binding site for its own SH2 domain. Located within the amino acid sequence between the SH2 domain and the kinase domain is a linker motif that interacts with Lyn's SH3 domain. These SH2 and SH3 domain interactions do not occlude the substrate-binding region of the kinase, but rather force an opening up of the kinase domain making it unfavourable to bind substrate/Mg-ATP. Activation of Lyn involves interaction with SH2 and/or SH3 domain binding motifs, which compete with Lyn's own SH3/SH2 domains, thus releasing the inhibitory conformation of the kinase domain. Lyn can then autophosphorylate on Y397 to generate a highly active enzyme that can then go on to phosphorylate specific substrates.
)