Guidance of Bidirectional Motor Complexes by mRNA Cargoes through Control of Dynein Number and Activity

Abstract

During asymmetric cytoplasmic mRNA transport, cis-acting localization signals are widely assumed to tether a specific subset of transcripts to motor complexes that have intrinsic directionality. Here we provide evidence that mRNA transcripts control their sorting by regulating the relative activities of opposing motors on microtubules. We show in Drosophila embryos that all mRNAs undergo bidirectional transport on microtubules and that cis-acting elements produce a range of polarized transcript distributions by regulating the frequency, velocity, and duration of minus-end-directed runs. Increased minus-end motility is dependent on the dosage of RNA elements and the proteins Egalitarian (Egl) and Bicaudal-D (BicD). We show that these proteins, together with the dynein motor, are recruited differentially to different RNA signals. Cytoplasmic transfer experiments reveal that, once assembled, cargo/motor complexes are insensitive to reduced cytoplasmic levels of transport proteins. Thus, the concentration of these proteins is only critical at the onset of transport. This work suggests that the architecture of RNA elements, through Egl and BicD, regulates directional transport by controlling the relative numbers of opposite polarity motors assembled. Our data raise the possibility that recruitment of different numbers of motors and regulatory proteins is a general strategy by which microtubule-based cargoes control their sorting.

Dynactin suppresses the retrograde movement of apically localized mRNA in Drosophila blastoderm embryos

“…Furthermore, we often observed that after reversal of direction, RNA particles move backward along the same exact path that they followed during minus end movement (Supplemental Movie 1). Taking these results together with previous published data from Bullock et al (2006), showing bidirectional movement of hairy transcripts in Drosophila blastoderm embryos, we conclude that the plus end runs of ftz transcripts reflect movement along MT tracks rather than diffusion of the cargo or the motor-cargo complex after detachment from the MTs.…”

A Dual Role for Actin and Microtubule Cytoskeleton in the Transport of Golgi Units from the Nurse Cells to the Oocyte Across Ring Canals

“…Investigating the role of cytoskeletal linkers such as Short stop, will be interesting, given that shot mutant egg chambers have a similar phenotype to dynein associated proteins Egalitarian and Bicaudal B (Rö per and Brown, 2003Brown, , 2004. In this study, we show that Golgi units as for mRNA such as grk (Clark et al, 2007), bcd (Mische et al, 2007), or hairy (Bullock et al, 2006), depend on dynein for transport from NCs to the oocyte. Interestingly, the average velocity of mRNA is faster than the transport of Golgi units (1.45 Ϯ 0.087 m/s (Clark et al, 2007) vs. 0.190 Ϯ 0.024 m/s, toward RC) and 0.25 Ϯ 0.036 m/s (Clark et al, 2007) vs. 0.110 Ϯ 0.026 m/s, through RCs).…”

“…To obtain further evidence for MTs involvement in RC transit, we sought to interfere with the MT function by blocking dynein, a minus-end-directed MT motor involved in the mRNA transport from the NC to the oocyte (Januschke et al, 2002;Tekotte and Davis, 2002;Duncan and Warrior, 2002;Bullock et al, 2006;Clark et al, 2007). Because null alleles of dynein heavy-chain (dhc) mutation compromise oocyte development (McGrail et al, 1995(McGrail et al, , 1997, we chose to disrupt dynein activity indirectly by overexpressing the dynactin subunit, dynamitin (Dmn; Burkhardt et al, 1997;Duncan and Warrior, 2002;Januschke et al, 2002).…”

“…Vesicles arrive much more quickly (1.7-fold greater) at the RCs than they do to cross it. The association to different motors and/or regulators could explain this difference (Bullock et al, 2006). 2) Trajectory specificity.…”

A Targeted Gain-of-Function Screen Identifies Genes Affecting Salivary Gland Morphogenesis/Tubulogenesis in Drosophila

“…Egl and BicD together with Dynein act as minus-end-directed microtubule motors, and as in most epithelial cells, the minus ends of microtubules are located near the apical surface in the salivary glands (Myat and Andrew 2002). hairy mRNA is one of the best-understood cargoes of Egl-and BicD-mediated transport (Bullock et al 2003(Bullock et al , 2006, and Hairy has been shown to be important for the regulation of apical membrane growth during salivary gland formation, in part through modulation of Crumbs (Myat and Andrew 2002). Thus, affecting hairy transcript localization through lowered levels of Egl and BicD could in turn affect the maintenance of apical membrane identity in the secretory cells.…”