Abstract
Author SummaryLike traffic on highways, molecular cargos are transported within cells on tracks that are collectively referred to as cytoskeletal networks. RNA molecules are one such cargo, and in many species, the localization of RNAs in egg cells or oocytes is essential for establishing the first asymmetries that are necessary for proper embryo development. RNAs can be actively transported by molecular motors that move cargos along the cytoskeletal tracks, but how such motors are capable of directing cargos to specific destinations within the cell is not yet known. Here we show that two motors, dynein and kinesin—known to carry out transport in opposite directions—are both directly involved in RNA localization in frog oocytes. To understand how these motors can promote directional cargo transport, we developed a system to monitor RNA transport in live oocytes. We find that the motor acting first in the pathway, dynein, is responsible for unidirectional transport. Bidirectional transport, mediated by kinesin, occurs subsequently on cytoskeletal tracks of opposing polarity near the RNA's final destination. Our results suggest a new model for directional transport comprising an initial directional cue that dominates over a later nondirectional step, acting to refine the ultimate cargo distribution.
Highlights
RNA transport underlies cell and developmental polarity in many organisms
Transport of RNA to the vegetal cortex of the Xenopus oocyte is mediated in part by kinesin motors [11,12], but other necessary steps in the transport pathway appear to be independent of kinesin
Given the mixed population of microtubules present within the vegetal cytoplasm [12], we investigated the role of cytoplasmic dynein in the vegetal RNA transport pathway
Summary
RNA transport underlies cell and developmental polarity in many organisms. Spatial regulation of gene expression mediated by subcellular RNA localization is required for embryonic axis formation, germ cell specification, and neuronal polarity [1,2]. In most models of mRNA localization, a single type of motor is bound to the RNA cargo to mediate localization. This simple model fails to account for cells that possess microtubule arrays of mixed polarity, or situations where the RNA cargo is capable of binding both plus- and minus-end directed motors. These issues raise the critical question of how RNAs are targeted to the correct subcellular location when they are capable of bidirectional transport
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