An RNA-binding tropomyosin recruits kinesin-1 dynamically tooskarmRNPs

Localization and local translation ofoskarmRNA at the posterior pole of theDrosophilaoocyte directs abdominal patterning and germline formation in the embryo. The process requires recruitment and precise regulation of motor proteins to form transport-competent mRNPs. We show that the posterior-targeting kinesin-1 is loaded upon nuclear export ofoskarmRNPs, prior to their dynein-dependent transport from the nurse cells into the oocyte. We demonstrate that kinesin-1 recruitment requires theDmTropomyosin1-I/C isoform, an atypical RNA-binding tropomyosin that binds directly to dimerizingoskar3’UTRs. Finally, we show that a small but dynamically changing subset ofoskarmRNPs gets loaded with inactive kinesin-1 and that the motor is activated during mid-oogenesis by the functionalized splicedoskarRNA localization element. This inefficient, dynamic recruitment of Khc decoupled from cargo-dependent motor activation constitutes an optmized, coordinated mechanism of mRNP transport, by minimizing interference with other cargo-transport processes and between the cargo associated dynein and kinesin-1.