Hypoxia Transiently Sequesters Mps1 and Polo to Collagenase-Sensitive Filaments in Drosophila Prometaphase Oocytes

BACKGROUND: The protein kinases Mps1 and Polo, which are required for proper cell cycle regulation in meiosis and mitosis, localize to numerous ooplasmic filaments during prometaphase in Drosophila oocytes. These filaments first appear throughout the oocyte at the end of prophase and are disassembled after egg activation. METHODOLOGY/PRINCIPAL FINDINGS: We showed here that Mps1 and Polo proteins undergo dynamic and reversible localization to static ooplasmic filaments as part of an oocyte-specific response to hypoxia. The observation that Mps1- and Polo-associated filaments reappear in the same locations through multiple cycles of oxygen deprivation demonstrates that underlying structural components of the filaments must still be present during normoxic conditions. Using immuno-electron microscopy, we observed triple-helical binding of Mps1 to numerous electron-dense filaments, with the gold label wrapped around the outside of the filaments like a garland. In addition, we showed that in live oocytes the relocalization of Mps1 and Polo to filaments is sensitive to injection of collagenase, suggesting that the structural components of the filaments are composed of collagen-like fibrils. However, the collagen-like genes we have been able to test so far (vkg and CG42453) did not appear to be associated with the filaments, demonstrating that the collagenase-sensitive component of the filaments is one of a number of other Drosophila proteins bearing a collagenase cleavage site. Finally, as hypoxia is known to cause Mps1 protein to accumulate at kinetochores in syncytial embryos, we also show that GFP-Polo accumulates at both kinetochores and centrosomes in hypoxic syncytial embryos. CONCLUSIONS/SIGNIFICANCE: These findings identify both a novel cellular structure (the ooplasmic filaments) as well as a new localization pattern for Mps1 and Polo and demonstrate that hypoxia affects Polo localization in Drosophila.