An RNA-binding tropomyosin recruits kinesin-1 dynamically tooskarmRNPs

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 precise recruitment and regulation of motor proteins to form transport-competent mRNPs.Using high-and super-resolution imaging, we determine the steps in motor recruitment to oskar mRNPs. We show that the posterior-targeting kinesin-1 is recruited upon nuclear export of oskar mRNPs, prior to their dynein-dependent transport from the nurse cells into the oocyte.We demonstrate that DmTropomyosin1-I/C is an atypical RNA-binding, nucleocytoplasmic shuttling Tropomyosin1 isoform that binds the oskar 3'UTR through recognition of a supramolecular RNA motif created upon dimerization of oskar molecules. Our data show that, in the oocyte, kinesin-1 is recruited by DmTropomyosin1-I/C to a dynamically changing, small subset of oskar mRNPs and is activated by the functionalized spliced oskar RNA localization element, revealing an ergonomic, coordinated mechanism of cargo transport. Highlights:-Drosophila Tropomyosin1-I/C is an RNA-binding, nucleocytoplasmic shuttling protein -DmTm1-I/C dynamically recruits Khc to oskar mRNPs -DmTm1-I/C preferentially binds an RNA motif formed upon dimerization of oskar 3' UTRs -The exon junction complex/spliced oskar localization element complex is endowed with kinesin activating function

Dynamic structural order of a low-complexity domain facilitates assembly of intermediate filaments

“…Why is proper deposition of germ granules at the posterior pole of Drosophila oocytes dependent on Tm1-I/C-specified intermediate filaments? Visualization of a GFP-tagged form of Tm1-I/C in living oocytes has shown that the protein becomes precisely restricted to the posterior tip of oocytes well before the completion of polar granule deposition (8). We speculate that assembled Tm1-I/C intermediate filaments localized to the posterior pole of fly oocytes might constitute a Velcro-like landing pad for one or more of the constituent RNA-binding proteins specifying polar granules.…”

“…Perplexingly, mutations proximal to the locus encoding the fly tropomyosin gene also impede the deposition of germ granules and subsequent formation of germ cells (6). These mutations interfere with the formation of a nonmuscle isoform of tropomyosin, designated Tm1-I/C, that is somehow required for germ cell specification (7,8).…”

Organizing the oocyte: RNA localization meets phase separation

“…In a second step, gurken RNA is subsequently transported to the dorsal-anterior corner of the oocyte along microtubules nucleated by the oocyte nucleus (MacDougall et al, 2003). Localization of oskar mRNA to the posterior of the oocyte relies instead on the plus-end directed microtubule motor, kinesin (Brendza et al, 2000;Cha et al, 2002;Gáspár et al, 2017;Januschke et al, 2002;Zimyanin et al, 2008). As will be discussed in detail below, RNAs are also transported by molecular motors along microtubule arrays to the vegetal pole of the Xenopus oocyte (Gagnon et al, 2013;Messitt et al, 2008).…”

RNA Granules Hitchhike on Lysosomes for Long-Distance Transport, Using Annexin A11 as a Molecular Tether

“…RNA granules have long been observed to traffic within neuronal axons and dendrites (Knowles et al, 1996;Gopal et al, 2017). While their transport requires both microtubules and motor proteins, how membraneless RNA granules are tethered to transport machinery remains incompletely understood (Clark et al 2007;Davidovic et al 2007;Dictenberg et al 2008;Dienstbier et al 2009;Dix et al 2013;Gá spá r et al 2017;Gagnon et al 2013).…”