Proper ER Morphology in the Drosophila Syncytial Embryo Depends on Reticulon-like Proteins. Zane J Bergman, Justin D Mclaurin, Amanda Q Sims, Blake Riggs. Biology, San Francisco State University, San Francisco, CA.

   Central to cell division is the correct and equal partitioning of genetic material to each daughter cell. Proper chromosome segregation requires the mitotic spindle, composed of microtubules and its associated proteins. Another important aspect of cellular division is the partitioning of the cytoplasm and organelles, which are needed for daughter cells to function. Unlike spindle events, the timing and mechanism of organelle segregation is not known. The endoplasmic reticulum (ER), an organelle essential for cellular function, exhibits a dramatic reorganization in mitosis and is also necessary for nuclear pore complex and nuclear envelope formation during mitotic exit. In the Drosophila melanogaster syncytial embryo, several rounds of rapid nuclear division occur without cytokinesis. Simultaneously, the ER is partitioned into discrete units that surround each nucleus. Recent evidence suggests that ER reorganization depends upon mitotic cyclin:CDK1 complex activity and a functioning APC/C, directly linking it to the same cell cycle events that control karyokinesis. The reticulon family of proteins has been shown in other organisms to be important for maintaining the morphology of ER structures. Here, we show that the reticulon-like protein 1 (Rtnl1) in Drosophila is responsible for correctly shaping the ER around the nucleus during mitotic events. Rtnl1 deficient embryos show defects in ER organization around the mitotic spindle and mitotic arrest with incorrectly shaped spindles. These mutations also negatively affect embryonic viability. Rtnl1 contains a Cdk1-consensus sequencethat may be the link between the cell cycle machinery and ER reorganization during mitosis. These data point to Rtnl1 being an important regulatory element that controls ER reorganization during mitosis.