Rtnl1 is enriched in a specialized germline ER that associates with ribonucleoprotein granule components

During oogenesis in Drosophila an organelle called the fusome plays a crucial role in germline cyst development and oocyte selection. The fusome consists of cytoskeletal proteins and intracellular membranes and, whereas many cytoskeletal components have been characterized, the nature and function of the membrane component is poorly understood. I have found the reticulon-like 1 (Rtnl1) protein, a membrane protein resident in the endoplasmic reticulum (ER), to be highly enriched in the fusome. In other Drosophila tissues Rtnl1 marks a subset of ER membranes often derived from smooth ER. During oogenesis, Rtnl1-containing membranes are recruited to the fusome by the cytoskeletal components and become concentrated into the forming oocyte. On the central part of the fusome, which is contained within the future oocyte and also at later stages in the growing oocyte and the nurse cells, Rtnl1-containing membranes colocalize with components of ribonucleoprotein complexes that store translationally repressed mRNAs. As the ER is actively transported into the oocyte, this colocalization suggests a role for the Rtnl1-containing subdomain in anchoring the ribonucleoprotein complexes within and/or transporting them into the oocyte.