Src kinase

The current understanding on activation of Tec kinases fits into a two-step model. In the first step an intramolecular interaction between the SH3 domain and aproline-rich region in the TH domain is disrupted by binding ofthe PH domain to phosphoinositides, G protein subunits, or the FERM domain of Fak. These interactions lead to conformational changes of Tec and translocation to the cytoplasmic membrane where, in a second step, Src kinases phosphorylate a conserved tyrosine residue in the catalytic domain thereby increasing Tec kinase activity. Autophosphorylation of a tyrosine residue in the SH3 domain further prevents the inhibitory intramolecular interaction resulting in a robust Tec kinase activation. 

Cabioglu N, Summy J, Miller C, et al. CXCL-12/stromal cell-derived factor-la transactivates HER2-neu in breast cancer cells by a novel pathway involving src kinase activation. Cancer Res 2005 65 6493-6497. 

Molecular insight into the protein conformation states of Src kinase has been revealed in a series of x-ray crystal structures of the Src SH3-SH2-kinase domain that depict Src in its inactive conformation [7]. This form maintains a closed structure, in which the tyrosine-phosphorylated (Tyr527) C-terminal tail is bound to the SH2 domain (Fig. 2). The x-ray data also reveal binding of the SH3 domain to the SH2-kinase linker [adopts a polyproline type II (PP II) helical conformation], providing additional intramolecular interactions to stabilize the inactive conformation. Collectively, these interactions cause structural changes within the catalytic domain of the protein to compromise access of substrates to the catalytic site and its associated activity. Significantly, these x-ray structures provided the first direct evidence that the SH2 domain plays a key role in the self-regulation of Src. 

Figure 2 Depiction of the active ( open ) and inactive ( closed ) conformations of Src kinase based on the analysis of x-ray structures of c-Src tyrosine kinase crystallized in its inactive state [7]. The stabilization of the inactive conformation is influenced by multiple events including intramolecular binding of the tyrosine-phosphorylated C-terminus tail to the SH2 domain as well as interactions between the SH3 domain and the SH2-kinase linker. CT, C-terminal NT, N-terminal. 

Many non-receptor tyrosine kinases have been identified as products of retrovirally encoded oncogenes. Non-receptor tyrosine kinases can be divided into two groups transmembrane and cytosolic families. The c-src tyrosine kinase is the prototype of the cytosolic tyrosine kinases. Regions within these non-receptor tyrosine kinases share homology with the Src kinase, known as Src homology 2 and 3 (SH2 and SH3) domains, and mediate protein-protein interactions between the receptor tyrosine kinases and the intracellular targets (reviewed in Cantley et al., 1991 Pawson and Gish, 1992 Mayer and Baltimore, 1993). 

Recently, luminacin C2 was isolated in a screen for Src kinase inhibitors. In-vitro experiments suggest that it elicits some of its biological effects via disruption of SH3-mediated association of any number of intracellular proteins with Src. See (a) S. Sharma, C. Oneyama, Y. Yamashita, H. Nakano, K. Sugawara, M. Hamada, N. Kosaka, T. Tamaoki, Oncogene 2001, 20, 2068-2079 (b) C. Oneyama, H. Nakano, S. Sharma, Oncogene 2002, 21, 2037-2050. 

Ali, D. W. and Salter, M. W. NMDA receptor regulation by Src kinase signalling in excitatory synaptic transmission and plasticity. Curr. Opin. Neurobiol. 11 336-342, 2001. 

The Src kinase Hck has been crystallized in its inactive form with the inhibitor PP1 complexed with it [26]. Similarly, Lck in its active form has been crystallized with the inhibitor PP2 [27]. 

These structures are currently guiding a number of laboratories in their attempt to design novel, more selective Src kinase inhibitors. 

Newman, M. and Josiah, S., Utilization of fluorescence polarization and time resolved fluorescence resonance energy transfer assay formats for SAR studies SRC kinase as a model system, /. Biomol. Screen., 9, 525, 2004. 

N-methyl derivatives on protein kinase C and src kinase activities, Biochem. 28 6796-6800. 

Figure 1. Ceramide mediated apoptosis signaling, Recent investigations hypothesize that, at least for drug induced apoptosis, the src kinase Lyn is activated and recruited to plasma membrane rafts. There it binds to and activates a neutral sphingomyelinase (SMase) within minutes. This initial rise in ceramide levels initiates an apoptotic cascade. Howevo- several cellular events can inhibit ceramide production (by blocking sphingomyelinase activation or clearing out ceramide accumulation through increased metabolisation) and therefore contribute to cellular resistance/survival.

Minshall RD, Timppathi C, Vogel SM, et al. Endothelial cell-surface gp60 activates vesicle formation and trafficking via G(i)-coupled Src kinase signaling pathway. J Cell Biol 2000 150(5) 1057-1070. 

P.A. Woods, V.L Jr. Adams, J.A. Phosphorylation driven motions in the COOH-terminal Src kinase, Csk, revealed through enhanced hydrogen-deuterium exchange and mass spectrometry (DXMS)./. Mol. Biol. 2002, 323, 871-881. 

Wong L., Lieser S., Chie-Leon B., Miyashita O., Aubol B., Shaffer J., Onuchic J.N., Jennings P., Woods V.L. Jr, Adams J.A. Dynamic coupling between the SH2 domain and active site of the COOH terminal Src kinase, Csk. J. Mol. Biol. 2004, 341, 93-106. 

Xu, Y., and S. Lindquist. Heat-shock protein hsp90 governs the activity of pp60v-src kinase. Proc Natl Acad Sci USA. 90 7074-8.1993. 

Novel Small-Molecule Inhibitors of Src Kinase for Cancer Therapy... 

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