ER shaping proteins guide spindle elongation and division during rapid cleavage mitoses

The ER is a complex network of membranes that inhabits much of the cytoplasm of cells - however, this network undergoes a massive condensation and rapid remodeling during cell division. In Drosophila cleavage divisions, this results in a tight association of the ER with centrosomes and mitotic spindle poles. Previous work has shown that this relationship between the ER and centrosomes must be finely tuned to enable successful spindle elongation, and that overaccumulation of the ER in these stages can result in failed centrosome maturation. During interphase, the ER exists in tubular and sheet-like arrangements, with a variety of "shaping" proteins enforcing these topologies. Here, we examine the contributions of these ER shaping proteins to the rapid changes that occur during cleavage mitoses in the Drosophila embryo. A screen of ER shaping proteins revealed that disruption of Reep-family proteins leads to mitotic failures at characteristic cleavage stages. Compromising ReepA, the Drosophila ortholog of the Reep1-4 subfamily, had a lesser impact on early embryonic mitoses. However, ReepB (the ortholog of the Reep5-6 subfamily) disruption, significantly affects ER mitotic coat morphologies, resulting in a frilled ER phenotype and a reduction of ER adherence to the spindle space accompanied by division failures. Overexpressing ReepA does not rescue ReepB mitotic or ER morphology defects and instead introduces local condensates of abnormal ER structures. These data suggest that dedicated Reep proteins guide ER mitotic properties at specific early developmental stages. Using a cell-based in vitro analysis of Drosophila Reeps, we identify differential "tubulating" properties of ReepA and ReepB. Together these data suggest that the minutes-scale ER remodeling required for early mitoses is governed by shaping proteins, and that ReepB family members are especially important in some of the most rapid cleavage divisions that occur in early embryo.