Signalling Pathways Operated by Receptor Protein Tyrosine Kinases
recorded at the time when receptor dimerization first came to light,42 has now been visualized by the use of fluorescent protein tagging.43 Different types of receptors can be recruited, so that PDGF receptors have been found associated with EGF receptors44 and these aggregates give rise to multiprotein signalling platforms that may contain numerous effectors.
Receptor dimerization, the concomitant conformational changes, and the prolonged juxtaposition are prerequisites for a successful transphosphorylation reaction. In the case of the insulin or PDGF receptor, the first phosphorylations occur in the activation segment (see page 782) and from here on the kinase
is activated, able to phosphorylate other substrates. Surprisingly, the EGFR (ErbB1) has only a single tyrosine phosphorylation site in the activation segment, but its phosphorylation is without effect on kinase activity. It appears that catalytic competence requires the formation of an asymmetric dimer, with one kinase domain activating the other. Cross-linking increases the probability of a productive interaction between the kinase domains.39,45
Assembly of receptor signalling complexes
The formation of signalling complexes and the subsequent signal transduction events have been studied in the following ways.
Measurement of enzyme activity, second messenger production and tyrosine-phosphorylation
The activated receptors for EGF and PDGF stimulate PLC . This results in the generation of DAG and IP3 and leads within seconds to the activation of protein kinase C and a rise in the concentration of cytosol free Ca2 .46–48 All of these can be measured. Furthermore, PLC itself becomes phosphorylated on tyrosine residues (see page 153) indicating that it interacts directly with the catalytic domain of the receptor. Activation of PDGF and insulin receptors also causes activation of phosphatidylinositol 3-kinase (PI 3-kinase).49 This phosphorylates PI(4,5)P2 forming PI(3,4,5)P3 (see Chapter 18). In addition,
a number of serine/threonine kinases are also activated. These include ribosomal S6-kinase (implicated in protein synthesis), C-Raf kinase (see below), and the mitogen-activated protein kinases (MAP kinase, see below). Most importantly, Ras becomes activated.50–52
Coimmunoprecipitation of proteins with activated receptors
To investigate the specific interactions of activated receptors, cells prelabelled with 35S-methionine are stimulated and then solubilized with detergent. The receptors, together with any associated proteins, are precipitated using an anti-receptor antibody. The associated proteins are detected by gel electrophoresis and autoradiography. Identification is achieved by microsequencing, immunoblotting, and other techniques. This
323
Signal Transduction
Yeast two-hybrid assay.
Transcription of the ectopic bacterial lacZ gene in engineered yeast utilizes the transcription factor GAL4 which codes for -galactosidase. In the assay, this factor
is substituted by two proteins: the DNAbinding segment of GAL4, fused to the protein of interest, and the transcription machinery-binding
segment of GAL4 fused to fragments of proteins derived from a cDNA library. Hundreds of yeast clones expressing different cDNAs are thus created. Those that exhibit -galactosidase activity (easily detected)
are selected and analysed for the inserted cDNA.59
was how the associations of protein tyrosine kinase receptors with Ras GAP, PLC , and PI 3-kinase were originally demonstrated.53,54 Using
a similar approach, but with cell lysates and purified receptors, it was shown that the p85 subunit of PI 3-kinase binds to the PDGF receptor.55
Detecting protein association in a cell-free system and cloning of receptor targets
Proteins expressed by a lambda-phage library in a bacterial host are screened for binding to the cytoplasmic domain of a receptor, labelled with 32P-phosphate. The relevant bacterial clones are identified using autoradiography and the DNA sequence of the phage insert is determined. This was how the association of the EGF receptor with the adaptor Grb2 and the p85-subunit of PI 3-kinase were discovered.56
Detecting protein association in microarrays
Large-scale protein microarrays have shown that phosphopeptide binding to SH2 or PTB domains occurs even when the peptide sequence does not match the consensus sequence for the binding site. Furthermore, it has become apparent that increasing the phosphopeptide concentration (the equivalent of increasing the expression levels of cell surface receptors) increases the variety of SH2/PTB domains that are recognized. This suggests that the level of receptor expression regulates not only the intensity of the signal, but also the range of effector proteins that are activated.24
Detecting protein–protein interaction in a yeast two-hybrid assay
This method has shown that insulin receptor substrate-1 (IRS-1) binds through its PTB domain to a phosphorylated tyrosine residue present in the juxtamembrane region of the receptor.57 The two-hybrid assay has also been instrumental in showing that H-Ras interacts directly with a conserved
81-residue segment at the N-terminus of the serine/threonine kinase B-Raf.58
Protein domains that bind phosphotyrosines and the assembly of signalling complexes
The assembly of receptor signalling complexes depends on protein–protein interactions that involve phosphorylated growth factor receptors. The first clues to the mechanism came from studies of p47gag-crk, a transforming protein devoid of any catalytic activity.60 It relies on the presence of a motif similar to a conserved region of Src (and many other tyrosine kinases), designated Src homology-2 (SH2) that interacts with tyrosine-phosphorylated proteins.60,61 Indirect evidence for a role of SH2 domains in interactions with phosphorylated growth factor receptors emerged with the observation that all proteins that operate downstream of these receptors, or that are involved in cell transformation, possess one or two copies of this domain. Significantly,
324