Egalitarian is a selective RNA-binding protein linking mRNA localization signals to the dynein motor

Abstract

Cytoplasmic sorting of mRNAs by microtubule-based transport is widespread, yet very little is known at the molecular level about how specific transcripts are linked to motor complexes. In Drosophila, minus-end-directed transport of developmentally important transcripts by the dynein motor is mediated by seemingly divergent mRNA elements. Here we provide evidence that direct recognition of these mRNA localization signals is mediated by the Egalitarian (Egl) protein. Egl and the dynein cofactor Bicaudal-D (BicD) are the only proteins from embryonic extracts that are abundantly and specifically enriched on RNA localization signals from transcripts of gurken, hairy, K10, and the I factor retrotransposon. In vitro assays show that, despite lacking a canonical RNAbinding motif, Egl directly recognizes active localization elements. We also reveal a physical interaction between Egl and a conserved domain for cargo recruitment in BicD and present data suggesting that Egl participates selectively in BicD-mediated transport of mRNA in vivo. Our work leads to the first working model for a complete connection between minus-end-directed mRNA localization signals and microtubules and reveals molecular strategies that are likely to be of general relevance for cargo transport by dynein.[Keywords: Drosophila; dynein; mRNA localization; microtubule-based transport; stem-loop] Supplemental material is available at http://www.genesdev.org.

Multiple Roles for Egalitarian in Polarization of the Drosophila Egg Chamber

“…grk mRNA, and to a lesser extent, osk mRNA, were also enriched in Egl pellets ( Figure 1D). Thus, as observed in Drosophila embryos (Dienstbier et al 2009;Vazquez-Pianzola et al 2014), Egl associates with mRNAs that are destined for localization.…”

“…As noted previously, Egl has been implicated in the Dyneindependent localization of mRNAs in the Drosophila embryo (Dienstbier et al 2009). We therefore investigated whether Egl might perform a similar function in the oocyte.…”

“…Egalitarian (Egl) is a Dynein adaptor that has been shown to link this motor with localized mRNAs in the Drosophila embryo (Navarro et al 2004;Dienstbier et al 2009). Within the ovary, the majority of studies have focused on the role of Egl in specification and maintenance of oocyte fate (Theurkauf et al 1993;Mach and Lehmann 1997;Huynh and St Johnston 2000;Bolivar et al 2001).…”

“…Egl binds directly to Dlc, a component of the Dynein complex (Navarro et al 2004), and in Drosophila embryos, Egl serves to link Dynein to mRNAs destined for localization (Dienstbier et al 2009). We therefore examined the localization of Dynein in Egl-depleted egg chambers using an antibody against Dhc.…”

“…However, oocyte fate is not maintained in these mutants, eventually resulting in cysts containing 16 nurse cells (Navarro et al 2004). In embryos, Egl has been shown to function as an adaptor for Dynein, by linking this microtubule motor to mRNAs destined for localization (Dienstbier et al 2009). Thus, Egl is required at very early stages of Drosophila egg chamber morphogenesis, and much later, during development of the embryo.…”