Role of BicDR in bristle shaft construction and support of BicD functions.

Cell polarization requires asymmetric localization of numerous mRNAs, proteins, and organelles. The movement of cargo towards the minus end of microtubules mostly depends on cytoplasmic dynein motors. In the dynein/dynactin/Bicaudal-D transport machinery, Bicaudal-D (BicD) links the cargo to the motor. Here we focus on the role of Drosophila BicD-related (BicDR) in the development of the long bristles. Together with BicD, it contributes to the organization and stability of the actin cytoskeleton in the not-yet chitinized bristle shaft. BicD and BicDR also support the stable expression and distribution of Rab6 and Spn-F in the bristle shaft, including the distal tip localization of Spn-F, pointing to the role of microtubule-dependent vesicle trafficking for bristle construction. BicDR supports the function of BicD and we discuss the hypothesis whereby BicDR might transport cargo more locally and BicD over long distances, such as to the distal tip. We also identified embryonic proteins that interact with BicDR and appear to be BicDR cargo. For one of them, EF1g, we showed that EF1g genetically interacts with BicD and BicDR in the construction of the bristles.