Adjunct Professor of Biochemistry and Biophysics
(PhD – University of California, San Diego)
HONORS AND AWARDS
Fellow of the UNC Academy of Educators, 2008-2013
Function and Regulation of MAP-kinase Activation Pathways in Saccaromyces Cerevisiae
Mitogen activated protein kinase (MAPK) cascades are a prevalent mechanism for intracellular signaltransmission. They are comprised of three sequentially acting protein kinases that are members of families known as MEK-kinases (MEKKs), MAPK/ERK kinases (MEKs) and mitogen activated kinases (MAPKs, also called extracellular regulated kinases, ERKs). MAPK cascades control distinct cellular responses to various different stimuli. In S. cerevisiae, separate cascades control mating and pseudohyphal differentiation, cell integrity and an osmotic stress response. The first MAPK activation cascade defined in humans is the one that mediates proliferation and differentiation in response to a large variety of mitogens. Two additional cascades in humans have been defined that control responses to physical and chemical stresses. Therefore, multiple cascades co-exist to control an array of distinct functions. Also, some of the enzymes are used in several pathways to mediate different responses. Despite the potential this situation presents for extensive cross talk between separate cascades, it is generally observed that MAPKs of a given pathway are activated in response to distinct sets of stimuli. Recent findings suggest this specificity is maintained because the enzymes of a given pathway are associated with each other in stable complexes.
The organizational paradigm for MAPK activation cascades comes from studies on the mating differentiation pathway of S. cerevisiae. The prototype enzymes for each of the three kinase families comprising the core enzymes of these conserved cascades were identified through their role in pheromone-induced mating differentiation. Subsequent genetic and biochemical analyses from our laboratory in collaboration with others defined the enzymatic activities of the core enzymes and revealed their order of function. The core enzymes of this cascade have stable interactions with each other and Ste5, a so-called scaffold protein. One role of the scaffold is to insulate the module and prevent illegitimate crosstalk between the mating differentiation and other MAPK modules. The associations with Ste5 may also be an important part of each activation step, but how the associations affect activities of the core enzymes is not yet known.
Our research continues to exploit well-defined MAPK activation pathways of S. cerevisiae to uncover the molecular mechanisms that govern protein-protein associations within a given module and to learn how these associations influence the mechanics of signal transmission. We are also interested in defining the mechanisms that allow for coordination of signaling through different pathways within the context of these insular modules. To address these issues we rely on biochemical approaches including phosphoprotein analyses and in vitro phosphorylation assays. We also apply recombinant DNA technology and mutagenesis approaches that allow the generation and analysis of variants with altered regulation or specificity. Given the high degree of conservation that has already been documented for these cascades, it is likely that the regulatory principles uncovered through our studies will be generally applicable.