Signal Transduction

A number of these mutated proteins operate in the early stages of tyrosine kinase signal transduction pathways. Cells may be transformed as a consequence of hypersecretion of growth factors, expression of variant forms of PTKs, over-expression of SH2/SH3-containing adaptor proteins, overexpression of serine/threonine protein kinases, or expression of variants of the small GTPases or their accessory proteins. At the downstream end of the signal transduction pathway, variants of transcription factors also act as potent cell transformers. Although tyrosine kinase phosphorylation accounts for only about 5% of total cellular phosphorylation activity, it has a key position in many signal transduction pathways and it is probably for this reason that the incidence of these genes in malignancy is so high (see Table 17.5).

A good example of cell transformation related to aberrant PTK activity is the role of JAK2 in acute lymphoblastic leukaemia (ALL). This is the most common childhood cancer of pre-B cell origin. These cells rely on paracrine or autocrine cytokine stimulation in order to proliferate and survive. Forms of this disease that are more resistant to chemotherapy express a constitutively activated mutant of JAK2.64 Apparently, this serves as a survival signal for these cells making them insensitive to the damage inflicted on DNA by chemotherapy which would normally induce apoptosis.65 Another example is Bcr-Abl, the cause of chronic myelogenous leukaemia (CML). This is a fusion protein caused by somatic reciprocal translocation between (paternal) chromosome 9 and (maternal) chromosome 22. The kinase activity of the fusion product, carried by Abl, is deregulated; we return to this topic in Chapter 23.

Essay: Non-receptor protein tyrosine kinases and their regulation

The nrPTKs comprise a large family with diverse roles in the control of cell proliferation, differentiation and death. Some are widely expressed; others are

Table 17.4  Families of non-receptor protein tyrosine kinases

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TCR, BCR, Soluble Tyrosine Kinases and NFAT

restricted to particular tissues. Their early classification was dominated by the discovery of pp60src, to the extent that the major group of kinases was simply known as the Src family. There are at least 10 known subfamilies of nonreceptor PTKs (see Figure 17.1 and Table 17.4).

The Src family kinases all share a similar structure. A unique chain at the N-terminus is followed by SH3 and SH2 domains (actually, the very archetypes of these domains that are so widely expressed) followed by a kinase domain and short tail at the C-terminus. Many Src kinases function by association with macromolecular signalling complexes assembled at membrane sites. Membrane targeting may be promoted by the unique N-terminal domain. Within the Src family, Src itself (pp60c–src), Fyn, Lyn, and Yes are myristoylated at the N-terminus. This provides the opportunity for membrane attachment that may be strengthened by palmitoylation at a nearby cysteine. Similarly,

Table 17.5  Components of tyrosine kinase signal transduction cascades manifested as cellular or viral oncogenes:  These oncogenes are gain-of-function mutants of the wild-type proteins, such as receptor tyrosine kinases, adaptor proteins, or guanine exchange factors. Serine or threonine protein kinases can also acts as oncogenes, but in comparison with tyrosine protein kinases their contribution is relatively modest

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Signal Transduction

534

TCR, BCR, Soluble Tyrosine Kinases and NFAT

Fig 17.13  Regulation of Src kinase activity by Csk and CD45.

Csk is recruited in the TRC signalling complex through interaction of its SH2 domain with the transmembrane phosphoprotein Cbp. Src is inactivated by Csk through tyrosine phosphorylation of its C-terminal

tail (1). A tyrosine phosphatase removes the phosphate from the tyrosine in the activation segment

(2). Inactive Src can be reactivated by CD45 which removes the phosphate in the C-terminus (Pc) (3).

Autophosphorylation of the activation segment completes the process (PA)

(4). This mode of regulation is a good example of how retro-control can limit activity. Active Src phosphorylates Cbp causing de-activation. Loss

of Src activity and subsequent dephosphorylation of Cbp re-initiates a cycle of Src activation. Many oncogenic tyrosine kinases have the effect of obliterating such oscillations because they are unable to adopt their inactive state.

localized in lipid rafts. The inhibitory action is linked to the removal of the phosphate from the activation segment of Src by a phosphatase of unknown identity. Activation of Src might then initially involve the dephosphorylation of Cbp by a tyrosine phosphatase, amongst which CD45 is a good candidate. CD45 also removes the phosphate from the pY530, thus allowing autoactivation of Src.

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List of abbreviations

Continued

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TCR, BCR, Soluble Tyrosine Kinases and NFAT

Continued

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