Identification of a nuclear export signal in the catalytic subunit of AMP-activated protein kinase

AMP-activated protein kinase (AMPK) is an energy sensor that regulates both cellular and organismal energy levels through phosphorylation, leading to the activation or inhibition of target proteins. AMPK functions as a heterotrimeric complex consisting of a catalytic alpha subunit and regulatory beta and gamma subunits. Until recently, research on AMPK was based on cell culture studies and the use of non-specific drugs. Since there are multiple isoforms of each subunit of AMPK, it was difficult to characterize AMPK's role due to genetic redundancy. In our lab, we identified the first null mutation of the AMPK alpha subunit in the fruit fly, Drosophila Melanogaster. Studying AMPK in fruit flies is somewhat ideal since all of its subunit functions and domains are conserved from flies to human and also since there are only one gene for each subunit of AMPK, overcoming the redundancy effect.;During my graduate study, we have characterized AMPK regulation in three distinct aspects. First, we identified AMPK's role in cell polarity and cell division. We demonstrated loss of AMPK activity causes loss of epithelial cell polarity and over proliferation under energetic stress. Second, we characterized AMPK's role at the whole organism level. Reduced AMPK causes hypersensitivity to starvation conditions and this causes a shortened life span and higher locomoter activity under energetic stress. In addition, loss of AMPK function causes inability to process and store lipids efficiently. Third, we identified a highly-conserved nuclear export sequence (NES) at the very C-terminal end of AMPKa. We showed that the loss of this sequence leads to increased nuclear localization. A transgenic fly expressing a truncated copy of AMPKalpha does not rescue the lethality or neuronal phenotype of AMPKalpha null mutant flies. A truncated copy also shows a reduced phosphorylation rate -- proposing AMPK's phosphorylation and activation takes place in the cytoplasm.