Mechanism and regulation of the protein kinase ERK2
The extracellular signal-regulated kinase 2 (ERK2) cascade plays important roles in a variety of cellular events such as proliferation, differentiation, and apoptosis. Involvement in such diverse cellular processes demands that this signal transduction pathway be strictly controlled. The fact that perturbations in the ERK2 signaling pathway are associated with a variety of diseases only further emphasizes the importance of maintaining the fidelity of the signaling cascade. In order to understand the mechanism of signal transduction fidelity, the kinetic mechanism and protein-protein interactions of ERK2 have been examined. In Chapter 1, a fluorescence anisotropy assay was developed to monitor the protein-protein interactions that occur between ERK2 and potential substrates. Results from these studies demonstrate that the phosphorylation status of ERK2 can alter protein-protein interactions. In Chapter 2, stopped-flow fluorescence spectroscopy studies demonstrate that the binding of EtsΔ138 to ERK2 follows a two-step mechanism in which binding occurs first followed by a conformational change. The results also suggest that substrate binding does not involve the active site, but instead occurs through the interaction of ERK2 docking-recruiting sites with EtsΔ138 docking motifs. In Chapter 3, transient kinetic methods were used to identify the second rate-limiting step in the ERK2 reaction pathway. The experiments provided evidence that an ADP dissociation step partially limiting enzymatic turnover. In Chapter 4, the magnesium effects on the catalytic mechanism of ERK2 were investigated. According to the study, ERK2 utilizes a second Mg2+ ion to facilitate ternary complex formation and catalysis, without inhibiting MgADP release, which is partially rate-limiting. The molecular basis for magnesium activation was found to stem from the interaction of the second Mg2+ ion by Asn-152. In Chapter 5, the interaction between ERK2 and PEA-15 was analyzed and PEA-15 was found to act as an inhibitor of ERK2- D-site interactions by binding to ERK2 via a proposed D-site motif. Further examination revealed that the neither the activation state of ERK2 nor the phosphorylation of PEA-15 has an affect on the affinity of these two proteins for one another.