Chapter 12
Signalling Pathways Operated by Receptor Protein Tyrosine Kinases
Introduction
Of the 90 genes that code for protein tyrosine kinases in the human genome, 58 are receptors (rPTK) and these are classified into 20
subfamilies. The remaining 32 are of the non-receptor type (nrPTK). Mouse homologues have been identified for nearly all of the human tyrosine kinases.1 Although absent from yeasts and protozoans,2 molecules related to the receptors for EGF and for insulin, that have integral catalytic domains, have been identified in marine sponges. It has been suggested that the insulin-receptor-like molecules evolved before the Cambrian Explosion (the name given to the seemingly rapid appearance of most of the major groups of complex animals, 530 million years ago) and contributed to the rapid emergence of the higher metazoan phyla.3 With respect to the transduction of signals from cell surface receptors, there are two main families of protein tyrosine kinases (PTKs). Here we consider the rPTKs that exist as integral
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Signal Transduction
The many abbreviations used in this chapter are collected together at the end of the chapter.
Since the phosphate– tyrosine bond is comparatively resistant to alkali, the detection of tyrosine phosphorylation was simplified by treating 32P-labelled cellular extracts with 1 M NaOH. More recently antibodies have been developed that specifically recognize individual phosphotyrosine epitopes, and this of course makes detection a very clean affair. The creation of antibodies of high specificity is now a business in its own right.
domains of transmembrane receptors. In Chapter 17 we discuss the nrPTKs which are present in the cytosol or are attached to the plasma membrane and can be recruited to receptors.
Spotting phosphotyrosine
Here is another turning point in science that owed as much to chance as to the application of a well-prepared mind. This involved Tony Hunter and the discovery of tyrosine phosphorylation of proteins associated with malignant transformation. He was interested in identifying the transforming antigens of the tumour-causing polyoma virus, of which the main component is the so-called middle T-antigen. A report that the src-gene product (v-Src) was associated with protein kinase activity4,5 begged the question whether other tumour virus gene products might also possess phosphorylating activities, and that this might underlie cell transformation. It became apparent that infection with the polyoma virus induces extensive phosphorylation of cellular protein, but that the transforming protein, middle T-antigen itself, also becomes phosphorylated. After proteolytic digestion of 32P-labelled protein into its component amino acids, the labelled residues were separated by electrophoresis. Unexpectedly, all of the label was confined to a new spot, now known to be due to phosphotyrosine (Figure 12.1).6
In a sense this discovery was accidental. It was common practice when carrying out electrophoretic procedures to re-use the buffers on subsequent
Fig 12.1 Separation of phosphotyrosine from phosphoserine and phosphothreonine by paper electrophoresis. Courtesy Tony Hunter.
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Signalling Pathways Operated by Receptor Protein Tyrosine Kinases
occasions. Eventually the pH must alter, the anodic buffer becoming more acidic, the cathodic more alkaline. Had the pH 1.9 electrophoresis buffer been freshly prepared, the separation of phosphotyrosine would not have occurred. In the event, with reuse, it had become more acidic (pH 1.7). The phosphotyrosine migrated more slowly and was separated from the phosphothreonine.
v-Src and other protein tyrosine kinases
With electrophoresis now carried out intentionally at pH 1.7, various other labelled protein digests were tested and it was found that v-Src can
phosphorylate tyrosine residues on a range of quite unrelated proteins. This identified v-Src as a protein tyrosine kinase and phosphorylation on tyrosine residues to be an authentic physiological process. Confirmation came with the finding that while labelling in non-transformed cells occurs almost exclusively upon serines and threonines, in cells transformed by Rous sarcoma virus
(see page 299), phosphorylated threonine, serine, and tyrosine residues are present in almost equal amounts.
It was also found that the transforming protein of the Abelson murine leukaemia virus (v-Abl) becomes labelled on a tyrosine residue when incubated in vitro with 32P-ATP.7 The target in this case is the tyrosine kinase itself, an example of autophosphorylation. Importantly, a second look at the phosphorylation status of the EGF receptor (see page 299) showed that the labelling due to stimulation occurs on tyrosine, not threonine residues as previously reported.8 The link between tyrosine phosphorylation and cell proliferation/transformation had been established.
Processes mediated through tyrosine phosphorylation
Tyrosine phosphorylation is not limited to the actions of the transforming viruses or growth factors. It regulates many other important signalling processes including:
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cell–cell and cell–matrix interactions through integrin receptors and focal adhesion sites9,10 (Chapter 13)
stimulation of the respiratory burst in phagocytic cells, such as neutrophils and macrophages11
activation of B lymphocytes by antigen binding to the B cell receptor12 activation of T lymphocytes by antigen-presenting cells through the T cell receptor complex13 (Chapter 17)
interleukin-2-mediated proliferation of lymphocytes14,15
activation of mast cells and basophils through the high-affinity receptor for IgE16
many developmental processes.
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