CANCER AND INTRACELLULAR PROTEINS
A wide variety of intracellular proteins bind with high affinity to activated RTKs via interaction domains that recognise phosphorylated tyrosines within specific peptide sequences (Fig. 1).
1.The EGFR interaction network.
The broad family of proteins that recognise phosphorylated tyrosines include ‘adaptors’ and enzymes, some of which also function as adaptors. Adaptor proteins generally do not have enzymatic activity but interact specifically with other proteins to mediate the formation of protein complexes or to draw proteins to specific locations. In essence, they act like bits of molecular Velcro. Two major types of domain are responsible for to phosphorylated tyrosines within specific peptide sequences: Src homology 2 (SH2) domains and phosphotyrosine binding (PTB) domains. The SH2 designation derives from homology with one of the domains of the SRC protein that contains two other domains, SH1 (the tyrosine kinase domain) and SH3. Other binding domains include Src homology 3 (SH3), which attaches to proline-rich sequences and pleckstrin homology (PH) domains, which bind to the charged head groups of phosphoinositides. In addition to adaptors and enzymes, some activated RTKs can recruit docking proteins that also undergo tyrosine phosphorylation to provide an additional ‘scaffold’ of multiple binding sites. The most familiar example of docking proteins is the insulin receptor substrate (IRS) family that contribute to signalling from insulin receptors.
An important example of adaptors are the three isoforms of the SHC (Src homology 2 domain containing) family that contain single SH2 and PTB domains and act in signal pathways from a variety of receptors including RTKs, antigen receptors, cytokine receptors, G- protein-coupled receptors and integrins. Activated SHC proteins have been associated with cancers and there is evidence that two forms of SHC are highly expressed in aggressive breast tumours.
Enzymes (other than the receptors themselves) are recruited to signalling pathways in exactly the same way as adaptor proteins – by having appropriate high-affinity binding sites. Thus, for example, signals activated by growth factors may be terminated by the cytosolic protein-tyrosine phosphatase PTPN6, which has two SH2 domains that permit binding to activated RTKs, permitting the dephosphorylation of adjacent tyrosines. The role of tyrosine phosphatases in suppressing tumour progression is not well understood but another of these cytosolic enzymes, PTPN12, does contribute to some types of primary breast cancers in which of function relieves constraint of a number of kinases, including EGFR and ERBB2.
In signal propagation, enzymes may be directly recruited to activated RTKs or form part of the downstream signalling pathways that are turned on. Phosphatidylinositol-specific phospholipase C gamma 1 (PLCG1) contains two SH2 and one SH3 domain in addition to its enzymatic domain and can be directly activated by RTKs to promote inositol 1, 4, 5-trisphosphate-mediated calcium signalling (Fig. 2).
2.SH2 family signalling proteins.
Like SHC proteins, PLCG1 is also highly expressed in some tumours. Whether directly activated or part of a downstream pathway, because enzymes are catalysts, once activated they continue carrying out their specific reaction until they are switched off. This feature means that when there is an enzyme in a pathway the signal is amplified at that point. Inbuilt amplification is a mechanism for increasing the sensitivity of the response and signal pathways often have a sequence (a cascade) of successive enzyme steps. The upshot of such pathways is the movement into the nucleus of phosphorylated proteins that can affect transcription.