The Mammalian Stress Response

Previous and Current research
The mitogen-activated protein kinases (MAPKs) are key components of cellular signalling pathways that are activated by growth factors and cellular stress. These pathways integrate diverse signals in order to elicit an appropriate physiological response. The latter include cell proliferation, transformation and apoptosis. The magnitude/duration of MAPK activation are important determinants of biological outcome and regulatory controls of MAPK activation are critical for both physiological and pathological cell functions. A family of at least ten dual-specificity MAP kinase phosphatases (MKPs) play a key role in the regulation of MAP kinase activity. These enzymes fall into two classes. The first, typified by CL100/MKP-1, are nuclear enzymes, which are encoded by growth factor and stress inducible genes. The second group, which includes Pyst1/MKP-3, are cytosolic enzymes and are not encoded by immediate early genes. Certain MKPs display substrate selectivity towards distinct MAP kinase isoforms. For example, Pyst1/MKP-3 inactivates ERK2 but not p38 or JNK MAPKs. The dephosphorylation of ERK2 by Pyst1/MKP-3 is accompanied by complex formation between the two proteins. Remarkably, this binding also results in the catalytic activation of Pyst1/MKP-3 in vitro and structural studies have revealed that this involves a conformational change within the active site of the phosphatase. Catalytic activation mirrors substrate selectivity in vivo, as p38 or JNK MAPKs were unable either to bind to or increase the catalytic activity of Pyst1/MKP3. Other MKPs including CL100/MKP-1 also undergo catalytic activation on binding to MAPKs indicating that this may be a general mechanism by which many members of this family of enzymes are regulated.

Future projects
We have gained important insights into the biochemical activities of the MKPs and the mechanisms which govern substrate recognition and specificity. However, we know very little about the regulation and physiological functions of these enzymes in mammalian cells and tissues. Our efforts in this area are directed towards obtaining mice which lack one or more of the genes within the Pyst subfamily of ERK-specific MKPs. We are also continuing our structure/function studies of the MKPs with particular emphasis on elucidating catalytic mechanisms and the nature of complexes formed between MKPs and MAPKs.