Pyruvate Kinase regulates Gurken translation by reduced TOR activity in Drosophila Melanogaster.

Abstract

Gurken (Grk) expression is required to specify the polarity of the developing oocyte during Drosophila oogenesis. Proper localization and translation of grk transcripts is required to achieve proper axis specification. Gkr translation initiation has been shown to be cap-dependent and require the activity of the DEAD-box RNA helicase, Vasa. Vasa activity can be repressed by the ATR/Chk2-dependent meiotic checkpoint when DNA double strand breaks (DSBs) persist in meiosis. Unrepaired DSBs in oocyte development of spindle-class mutants activate this checkpoint and result in inefficient grk translation and loss of dorsal fates. This inefficient grk translation is thought to be related to reduced Vasa activity. In a screen for suppressors of the ventralized eggshell phenotype seen in spindle-BBU mutants, we identified a mutation in the PyK gene. We show that PyK mutations suppress the eggshell phenotype independent of the DSB repair delay and Vasa phosphorylation seen in spn-B mutants. This suggests that the eggshell phenotype is corrected by overcoming the translational block of grk transcripts seen in spindle mutants. PyK has been identified as a member of the TOR signaling pathway. Direct inhibition of the TOR kinase with rapamyacin suppresses the ventralized eggshell phenotype in spn-B mutant females. PyK modulates TOR kinase activity through the TSC1/2 heterodimer. During dietary starvation, TOR activity promotes capdependent translation by restraining the activity of the translation inhibitor eIF4E binding protein (4EBP). We hypothesize that reduced TOR activity promotes grk translation independent of the ATR/Chk2 meiotic checkpoint pathway. Recent data indicates that this may be achieved by way of IRES-dependent translation initiation of grk when TOR activity is low. This discovery suggests flies are able to maintain the translation of developmentally important transcripts such as grk during periods of nutrient limitation.