An RNA ‐binding atypical tropomyosin recruits kinesin‐1 dynamically to oskar mRNP s

Abstract

Localization and local translation of oskar mRNA at the posterior pole of the Drosophila oocyte directs abdominal patterning and germline formation in the embryo. The process requires recruitment and precise regulation of motor proteins to form transport‐competent mRNP s. We show that the posterior‐targeting kinesin‐1 is loaded upon nuclear export of oskar mRNP s, prior to their dynein‐dependent transport from the nurse cells into the oocyte. We demonstrate that kinesin‐1 recruitment requires the Dm Tropomyosin1‐I/C isoform, an atypical RNA ‐binding tropomyosin that binds directly to dimerizing oskar 3′ UTR s. Finally, we show that a small but dynamically changing subset of oskar mRNP s gets loaded with inactive kinesin‐1 and that the motor is activated during mid‐oogenesis by the functionalized spliced oskar RNA localization element. This inefficient, dynamic recruitment of Khc decoupled from cargo‐dependent motor activation constitutes an optimized, coordinated mechanism of mRNP transport, by minimizing interference with other cargo‐transport processes and between the cargo‐associated dynein and kinesin‐1. image Polarized localization of oskar mRNP s in the Drosophila oocyte depends on proper coordination of motor protein assembly and mRNA processing. Uncoupling recruitment of an inactive kinesin and its activation by   mRNA splicing ensures subsequent  oskar  mRNP targeting to the posterior pole. Drosophila Tropomyosin1‐I/C is an RNA ‐binding atypical tropomyosin. Tropomyosin1‐I/C preferentially binds an RNA motif formed upon dimerization of oskar 3′ UTR s. Tropomyosin1‐I/C inefficiently but dynamically recruits inactive kinesin heavy chain to oskar mRNP s upon their nuclear export. The exon junction complex/spliced oskar localization element complex activates kinesin‐1 during mid‐oogenesis.