Identification of a Nuclear Export Signal in the Catalytic Subunit of AMP-activated Protein Kinase

The metabolic regulator AMP-activated protein kinase (AMPK) maintains cellular homeostasis through regulation of proteins involved in energy-producing and -consuming pathways. Although AMPK phosphorylation targets include cytoplasmic and nuclear proteins, the precise mechanisms that regulate AMPK localization, and thus its access to these substrates, are unclear. We identify highly conserved carboxy-terminal hydrophobic amino acids that function as a leptomycin B-sensitive, CRM1-dependent nuclear export sequence (NES) in the AMPK catalytic subunit (AMPK alpha). When this sequence is modified AMPK alpha shows increased nuclear localization via a Ran-dependent import pathway. Cytoplasmic localization can be restored by substituting well-defined snurportin-1 or protein kinase A inhibitor (PKIA) CRM1-binding NESs into AMPK alpha. We demonstrate a functional requirement in vivo for the AMPK alpha carboxy-terminal NES, as transgenic Drosophila expressing AMPK alpha lacking this NES fail to rescue lethality of AMPK alpha null mutant flies and show decreased activation loop phosphorylation under heat-shock stress. Sequestered to the nucleus, this truncated protein shows highly reduced phosphorylation at the key Thr172 activation residue, suggesting that AMPK activation predominantly occurs in the cytoplasm under unstressed conditions. Thus, modulation of CRM1-mediated export of AMPK alpha via its C-terminal NES provides an additional mechanism for cells to use in the regulation of AMPK activity and localization.