Systematic analysis of atx-2 suppressors reveals a novel regulator of PAR-5/14-3-3sigma function during mitosis in Caenorhabditis elegans

RNA regulation plays a critical role in mitosis, yet the mechanisms remain unclear. Our lab recently identified that the conserved RNA-Binding Protein (RBP), ATX-2, regulates cytokinesis by regulating the targeting of ZEN-4 to the spindle midzone through a conserved translation regulator, PAR-5/14-3-3sigma (Gnazzo et al., 2016). While co-depletion of ATX-2 and PAR-5 restored ZEN-4 targeting to the spindle midzone, it did not rescue cell division. To identify factors that may work in concert with ATX-2 to regulate cell division, we conducted a two-part, candidate RNAi suppressor and visual screen to identify factors that are important for cell division and also mediate the targeting of ATX-2 to the centrosomes and the spindle midzone. Using this approach, we identified ten genes that suppress the embryonic lethality defect observed in atx-2 mutant embryos. These ten genes, including act-2, cgh-1, cki-1, hum-6, par-2, rnp-4, vab-3, vhl-1, vps-24, and wve-1, all have some role regulating RNA or the cell cycle. Five of these genes (cgh-1, cki-1, vab-3, vhl-1, vps-24) fail to target ATX-2 to the centrosomes and midzone when depleted. The strongest suppressor of the atx-2 phenotype is the DEAD-box RNA helicase CGH-1/DDX6, which has been implicated in cell division, RNA processing and translation, and neuronal function. Loss of CGH-1 rescued the cytokinesis defect and also restored ZEN-4 localization to the spindle midzone. ATX-2 and CGH-1 are mutually required for their localization to centrosomes and the spindle midzone. Our findings provide the first functional evidence that CGH-1/DDX6 regulates ATX-2 function during mitosis to target ZEN-4 to the spindle midzone via PAR-5/14-3-3sigma. We suggest that RNA machinery is necessary for the completion of cytokinesis.