Akt Phosphorylation of Drosophila Heat-Shock Factor: A Signature for Stress Resistance

The heat-shock response is vital to cellular homeostasis. Drosophila melanogaster heat-shock factor (dHSF) is the primary transcriptional activator in the stress response pathway for induction of heat-shock-mediated gene transcription. This work investigates the potential for dHSF to undergo post-translational modification by phosphorylation and lysine tagging, specifically, direct phosphorylation by kinases and covalent-lysine tagging by ubiquitin, acetyl, and SUMO groups. Direct phosphorylation of, and binding to, dHSF was demonstrated by Akt/PKB kinase. Knock-down of this kinase by RNAi resulted in a heat-shock phenotype for dHSF and the acquired DNA-binding ability characteristic of activated transcription factor. Site-directed mutagenesis of lysines within a putative nuclear localization sequence (NLS) revealed two potential sites for regulation of dHSF activation by post-translational modification. The functional consequences of synergistic Akt phosphorylation and lysine modifications are discussed – this work implicates a role for direct kinase phosphorylation in regulating the stability of dHSF