Cytosolic tyrosine-kinases

III. Tyr protein kinases A. Cytosolic tyrosine kinases src, fgr, abl, etc.) B. Receptor tyrosine kinases (RTKs) Plasma membrane receptors for hormones such as epidermal growth factor (EGF) or platelet-derived growth factor (PDGE) Raf (a protein kinase)... 

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). 

The ligands of the receptor tyrosine kinases are generally multivalent. The multiple binding sites of the ligand enable it to orient two (or more) of the receptor subimits so that their cytosolic tyrosine kinase activity is stimulated. 

Fig. 3.3 The structure of the N-terminal SH3 domain of Grb2 bound to a proline-rich Sos peptide has been determined by NMR.29.30 The structure of the Gbr2 N-terminal SH3 domain, compiexed with a 10-residue peptide, comprising residues 1134-1144 (VPPPVPPRRR-NHz) of Sos, is shown. The prolyl residues, P2, P3, P6, and P7, which interact with the SH3 domain of Grb2 are marked. (The ribbon model was reproduced with permission of the authors and J. Mol. Biol, from data in ref. 30, available In databanks.) A variation of this scheme is the recognition of a proline-rich sequence (APTMPPPLPP) in the GAP protein for Rho by the SH3-domain of the cytosolic c-Abi tyrosine kinase. i This interaction couples the Rho/GAP tightly to this cytosolic tyrosine kinase and brings the momomeric G protein, Rho, under the control of phosphorylation by the kinase.

Thanks to crystal structures of human c-Src and its close relative, the hemopoietic cell kinase, Hck, the control of the activity of these cytosolic tyrosine kinases by phosphorylation dephosphorylation is now understood (Fig. 3.7 and Plate 5). 

Another common signalling route is the phosphaddylinositol 3-kinase pathway. Class I Ptdins 3-kinases, PI 3-kinases, are heterodimers, consisting of a catalytic and a regulatory subunit. Regulatory subunits of the Class Ia Ptdins 3-kinases (the p85 subunits) serve as adaptors and bind cytosolic tyrosine kinases throu their SH2 and SH3 domains (Table 4.1). 

Signalling in T and B cells is controlled by phosphorylation and dephosphorylation, whereby Src-related cytosolic tyrosine kinases play a major role. 

The role of cytosolic tyrosine kinases in the pathogenesis of human leukaemia has made them a target for drugs. The so-called Tyrphostins , developed by A. Levitzki and... 

Src is a oncogene first found in the Rous sarcoma virus where it encodes the pp60 kinase. The corresponding cellular kinase is pp60 -s =. The Src kinases are regulated by phosphorylation de-phosphorylation. Members of the Src family of cytosolic tyrosine kinases with important funtions in cell signalling are for example Yes, Fyn, Lck, Blk, Btk, Csk, Hck, Fgr and Yrk. Fyn associates with the p85 subunit of 1-phosphatidyl inositol... 

Plate 3 A view of the crystal structure of the SH3 domain of the human, cytosolic tyrosine kinase Fyn, complexed with a proline-rich SOS peptide (see ref. 35 of Chapter 3). The picture presented shows that a SH3 domain is a globular structure of approximately 60 amino acids with a well-conserved /3-sheet Structure (see ref. 36 of Chapter 3). (This picture was generously contributed by Dr Martin Noble, University of Oxford.)... 

PBKs phosphorylate inositol lipids at the 3 position of the inositol ring to generate the 3-phosphoinositide PI(3,4,5)P3, (PIP3). PBKs of class IB are directly activated by GP t, but others PBK also belong to pathways of receptors with or coupled to tyrosine kinase activity. PIP3 may recruit to tlie membrane various protein kinases, phosphoinositide-dependent kinase-1 (PDK-1), protein kinase B (PKB, or Akt), protein kinase (PKCQ, PLC t and cytosolic tyrosine kinases such as the Bruton s tyrosine kinase (Btk). 

Inhibition of EGF-R tyrosine kinase by hypericin 1 was shown to be irreversible, non-competitive and time as well as temperature dependent. The IC50 increased from 0.75 pM in the dark to 44 nM with light illumination for 30 min. This effect was presumably due to a type I photosensitization mechanism since exclusion of oxygen did not alter the inhibition curve. Some Ser/Thr protein kinases (e.g., protein kinase A, casein kinase 1 and 2) and the enzyme 5 -nucleotidase were not inhibited even at concentrations > 100 pM [144]. However, the same authors recently reported that hypericin 1 in addition to protein kinase C also caused the light-dependent inhibition of certain other Ser/Thr kinases (e.g. protein kinase CK-2, mitogen-activated kinase) and the insulin receptor tyrosine kinase, while it was ineffective towards the cytosolic tyrosine kinases Lyn, Fgr, TPK-IIB and CSK. These results suggest that distantly related protein kinases could still share common reactive domains for the interaction with hypericin 1 [156]. In contrast to the above mentioned studies, Richter and Davies [157] observed no inhibition of EGF-induced tyrosine phosphorylation of the EGF-R in HN5 squamous carcinoma... 

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