Mechanisms of Cellular Response to Environmental Changes

     Research in the laboratory is centred around mechanisms that underpin cellular response(s) to changes in the environment and phases of growth or life cycle.  The research has two quite distinct components: the one uses Saccharomyces cerevisiae as a model eukaryotic system to evaluate the cellular response to heavy metal exposure, such as Copper, Chromium (VI) and Cadmium.  The other, in collaboration with Professor P.C.Tai (also in the Dept. of Biology at GSU) investigates the various roles of SecA in protein tranlsocation in prokaryotes


Oxidative Targeting of Enzymes in S. cerevisiae: While both cellular response mechanisms differ dramatically in their mechanisms of action, they each share a significant metabolic component.  The first level response of S. cerevisae to exposure to sub-lethal concentrations of heavy metals involves targeted oxidation of enzymes predominantly within the glycolytic pathway (see Figure 1).  

    Such targeting is believed to result in an immediate -but transient- decrease in glycolytic enzyme activity, possibly resulting in a catabolic shift of carbon flow away from glycolysis and into the pentose-phosphate pathway -to generate NADPH2 that can be utilized to combat the oxidative insult.  Work in the lab is presently focused upon this and other cellular responses to heavy metals that affect protein expression; namely transcriptional levels of key enyzmes and regulatory proteins.

Figure 1: The Glycolytic and Pentose Phosphate pathways in S. cerevisiae, showing the key enzymes and reactions (in red) that appear to be specifically targeted for oxidation in response to the presence of heavy metals, such as Cu, Cd and Cr.  



Roles of the multifunctional SecA protein in protein translocation: SecA is a key component of the translocation machinery in bacteria and, while it has no direct analogue in eukaryotes, provides a number of essential roles in protein translocation in the cell; from forming a requisite ATPase in the SecA-YEG transporter complex that drives protein precursors with signal peptides through the SecYEG protein translocation channels, to forming an additional "SecA-only" channel within the cytoplasmic membrane that facilitates the translocation of signal free proteins across the cytoplasmic membrane -albeit with less efficiency than the SecA-YEG channels (Figure 2). Expression of SecA protein is also distinct from that of all the other members of the translocation machinery, and has been found to be differentially expressed at various phases of cellular growth in E. coli and Bacillus sp. As such, it is believed that the role of SecA itself may vary as a functon of these growth phases. An analysis of the ubiquitous, transloaction role for SecA forms the basis of reseach in this laboratory, in strong collaboration with P.C. Tai, who has been a pioneer in this area of research for the last forty years and more..




     Figure 2.  Some of the proposed multi-protein complexes that SecA is believed to form with itself -as a soluble SecAs dimer or in the membrane as SecA-only channel, with SecYEG,DF and with the 70S ribosome.




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