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Thesis (Ph. D.)--University of Texas at Austin, 2004. / Supervisor: Kevin N. Dalby. Vita. Includes bibliographical references.

Mitogen-activated protein kinases (MAPKs) are an important subfamily of protein kinases that are well conserved in all eukaryotes. MAPKs are the final component of a three-tiered signaling module that regulates the activation of various essential cellular responses. They activate most of their substrates through catalyzing their phosphorylation. However, emerging evidence reveals that some MAPKs also possess non-catalytic functions. In particular, the human MAPK ERK2 can bind to DNA directly and mediate gene expression. The mechanism by which ERK2 binds to DNA is still unclear. In this work, we combined structural, biophysical and biochemical methods to confirm DNA binding by ERK2 and to investigate whether ERK2’s closest plant homolog MPK4 also binds to DNA. First, we identified a possible ERK2-like DNA consensus motif in plant MAPKs. We found that several plant MAPKs, including MPK4, harbor a basic motif (KARK/R or ARR/K) in a region corresponding to the ERK2 KAR motif reported to mediate DNA binding. Next, we determined the DNA binding affinity of ERK2 and MPK4 to different DNA fragments and found that MPK4 associated directly with DNA in vitro, albeit with a significantly lower affinity than did ERK2. Moreover, we observed that ERK2 and MPK4 showed preferred binding to different DNA sequences. Site-directed mutagenesis on the proposed DNA binding region of MPK4 greatly weakened DNA binding, confirming that MPK4 and ERK2 use the same structural elements to associate with DNA. Phosphorylation of the MAPKs through an upstream MKK affected the DNA binding capacity for both ERK2 and MPK4, although the effects differed. Lastly, we observed that a MPK4 mutant with a constitutively increased catalytic affinity displayed a markedly stronger DNA binding affinity compared to wild type MPK4 and phosphorylated MPK4. By demonstrating that the plant MPK4 associated with DNA in vitro, and that this association can be modified by phosphorylation and mutations, we open the possibility of additional kinase-independent functions in plant MAPKs.

Skeletal muscle function is important to the human body for daily activities. Mechanical signals are critical to the maintenance of that function. Muscle diseases, such as the muscular dystrophies, in which the force transmission apparatus is compromised, have devastating effects on muscle function and quality of life. Mechanical signals activate intracellular signaling to maintain function. ERK2 has been shown to be quickly and strongly upregulated following stretch, leading to cell proliferation. Stretch has been shown to cause deformation of caveolae, invaginations of the plasma membrane that inhibit ERK signaling. This leads to the hypothesis that stretch induced deformation of caveolae may initiate mechanotransduction by activating ERK2. Reducing caveolin-3 expression via siRNA knockdown eradicated the stretch-induced effect on ERK2 activation, indicating that caveolin is required for the stretch response. Stabilizing caveolae structure by temperature reduction or destabilizing caveolae by cholesterol depletion resulted in changes consistent with the hypothesis that proper caveolae structure plays an important role in inhibition of signaling molecules and that deformation mediates mechanotransduction, resulting in changes in activation of ERK2.

Dictyostelium discoideum is a simple model widely used to study many cellular functions, including differentiation, gene regulation, cellular trafficking and directional migration. Adaptation mechanisms are essential in the regulation of these cellular processes. The misregulation of adaptation components often results in persistent activation of signaling pathways and aberrant cellular responses. Studying adaptation mechanisms regulating cellular migration will be crucial in the treatment of many pathological conditions in which motility plays a central role, such as tumor metastasis and acute inflammation. I will describe two adaptation mechanisms regulating directional migration in Dictyostelium cells. The Extracellular signal Regulated Kinase 2 (ERK2) plays an essential role in Dictyostelium cellular migration. ERK2 stimulates intracellular cAMP accumulation in chemotaxing cells. Aberrant ERK2 regulation results in aberrant cAMP levels and defective directional migration. The MAP Phosphatase with Leucine-rich repeats (MPL1) is crucial for ERK2 adaptation. Cells lacking, MPL1 (mpl1- cells) displayed higher pre-stimulus and persistent post-stimulus ERK2 phosphorylation, defective cAMP production and reduced cellular migration. Reintroduction of a full length Mpl1 into mpl1- cells restored aggregation, ERK2 regulation, random and directional motility, and cAMP production similar to wild type cells (Wt). These results suggest Mpl1 is essential for proper regulation of ERK2 phosphorylation and optimal motility in Dictyostelium cells. Cellular polarization in Dictyostelium cells in part is regulated by the activation of the AGC-related kinase Protein Kinase Related B1 (PKBR1). The PP2A regulatory subunit, B56, and the Glycogen Synthase Kinase 3 (GSK3) are necessary for PKBR1 adaptation in Dictyostelium cells. Cells lacking B56, psrA-cells, exhibited high basal and post-stimulus persistent phosphorylation of PKBR1, increased phosphorylation of PKBR1 substrates, and aberrant motility. PKBR1 adaptation is also regulated by the GSK3. When the levels of active GSK3 are reduced in Wt and psrA- cells, high basal levels of phosphorylated PKBR1 were observed, in a Ras dependent, but B56 independent mechanism. Altogether, PKBR1 adaptation is regulated by at least two independent mechanisms: one by GSK3 and another by PP2A/B56.

We worked on finding a new kinase regulator to develop basic data to be used for cancer prevention. Our work found a link between three previously unrelated proteins involved in cancer, ERK2 and ERK6 and Scribble. The MAP kinase cascade is involved in cell proliferation, which is highly deregulated in cancer. Through the screening of ERK6 associating molecules, we found the cell polarity, and cell cycle related molecule Scribble. Furthermore, we found that Scribble was a dual-specific kinase regulator. We clearly demonstrated that these ERKs interact with Scribble through the LRR and the PDZ domains of Scribble. We hypothesize that Scribble may function as a scaffolding protein for ERK2 and ERK6, since Scribble has been found to down regulate the kinase activity of these ERKs.

Extracellular signal-regulated kinases (ERKs) modulate cellular activities in response to extracellular stimuli and play important biological roles. Thus, perturbed kinase pathways induce pathological conditions, such as tumor development. Rit, a novel member of the Ras family GTPases, activase ERK6, and its over-expression confers tumorigenicity. We hypothesized the presence of scaffolding molecules specific to ERK6, similar to other known MAP kinases. We performed yeast two-hybrid assays using ERK6 as bait, and Scribble was identified as a binding partner. Scribble contains 16 LRR domains and four PDZ domains. We performed immunoprecipitation (IP) assays and discovered ERK2 as another binding partner. Surprisingly, no interaction was observed with the highly homologous MAP kinase, ERK1. No other representative kinases showed binding capabilities with Scribble. IP data confirmed that both ERK2 and ERK6 bind to Scribble through its LRR and PDZ domains. Deletion of ten aminoi acids from the C-terminus of ERK2 and ERK6 abolished these interactions. In vitro kinase assays indicated the kinase suppressing ability of Scribble. Focus formation assays were performed with RitQ79L and H-RasV12 as constitutive activators of ERK6 and ERK2, respectively, in the presence of Scribble. Results confirmed the role of Scribble as a tumor suppressor.

Protein kinases ERK1 and ERK2 are one of the most studied proteins in cell signalling. Both proteins are involved in a plethora of processes, such as phosphorylation and activation of kinases as part of signalling pathways. Enzymes ERK1 and ERK2 are part of MAPK/ERK signalling cascade, connected to many cellular including cell proliferation, cell growth or differentiation. The MAPK/ERK signalling cascade is often activated in different types of tumors, making it a candidate for developing new chemical inhibitors. One of the important questions in fundamental research of ERK1 and ERK2 protein kinases is the search for difference between these proteins. Current knowledge points to redundancy of both proteins, howver several examples suggest otherwise. Recently, the work presented in Casanova et al. 2012 indirectly suggests divergent effect of ERK1 and ERK2 on cap-independent translation initiation. In the Laboratory of RNA biochemistry we focus on HCV IRES (Hepatitis C Virus Internal Ribosome Entry Site) dependent translation initiation. This diploma thesis lead to establish RNA interference method in our laboratory and to establish reporter system to study ERK1 and ERK2 effect on HCV IRES dependent translation initiation. Based on our data acquired during our research, we present in this work...

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