Direct Observation of Regulated Ribonucleoprotein Transport Across the Nurse Cell/Oocyte Boundary

Abstract

In Drosophila, the asymmetric localization of specific mRNAs to discrete regions within the developing oocyte determines the embryonic axes. The microtubule motors dynein and kinesin are required for the proper localization of the determinant ribonucleoprotein (RNP) complexes, but the mechanisms that account for RNP transport to and within the oocyte are not well understood. In this work, we focus on the transport of RNA complexes containing bicoid (bcd), an anterior determinant. We show in live egg chambers that, within the nurse cell compartment, dynein actively transports green fluorescent protein-tagged Exuperantia, a cofactor required for bcd RNP localization. Surprisingly, the loss of kinesin I activity elevates RNP motility in nurse cells, whereas disruption of dynein activity inhibits RNP transport. Once RNPs are transferred through the ring canal to the oocyte, they no longer display rapid, linear movements, but they are distributed by cytoplasmic streaming and gradually disassemble. By contrast, bcd mRNA injected into oocytes assembles de novo into RNP particles that exhibit rapid, dynein-dependent transport. We speculate that after delivery to the oocyte, RNP complexes may disassemble and be remodeled with appropriate accessory factors to ensure proper localization.

Microtubule–microtubule sliding by kinesin-1 is essential for normal cytoplasmic streaming in Drosophila oocytes

“…Organelle transport by kinesin-1 along these subcortical microtubule arrays has been proposed to drive ooplasmic streaming, by exerting hydrodynamic forces on the surrounding cytoplasm (8,52). However, surprisingly, the general kinesin-1 cargo adapter KLC and the microtubule minus-end motor dynein are dispensable for fast ooplasmic streaming (7)(8)(9). This independence of KLC and dynein is reminiscent of another function of kinesin-1, microtubule-microtubule sliding, which is also KLC-and dyneinindependent (10).…”

“…Cytoskeletal filaments and molecular motors are major players that generate the forces required for these activities. For instance, kinesin-1 and microtubules have been shown to provide the mechanical forces required in multiple cellular contexts, such as organelle transport (1)(2)(3)(4), ooplasmic streaming (5)(6)(7)(8)(9), and process formation (10)(11)(12)(13)(14)(15).…”

“…It has been proposed that streaming of ooplasm is driven by the directional transport of cargoes along microtubules by kinesin-1 (8). Surprisingly, kinesin-1 light chain (KLC) is dispensable for streaming (7,9), even though KLC is used as the adapter for many of the cargoes transported by kinesin-1. Based on these results, it has been proposed that kinesin-1 transports vesicles to drive ooplasmic streaming by an unknown KLC-independent mechanism (7).…”

“…Based on these results, it has been proposed that kinesin-1 transports vesicles to drive ooplasmic streaming by an unknown KLC-independent mechanism (7). In contrast to kinesin-1's function in streaming, dynein is not required for fast streaming; instead, it inhibits the shift from slow streaming to fast streaming (8,9). This KHC-dependent and KLC/dynein-independent streaming mechanism highly resembles another important function of KHC, called microtubule-microtubule sliding, studied by our group (10)(11)(12)(13)(14)(15).…”

Characterization of kinesin-like proteins in silkworm posterior silkgland cells

“…In Drosophila, DmKHC is required for posterior localization of oskar mRNA [44]. However, in Drosophila oocytes, dynein, but not kinesin, has been reported to actively transport exuperantia, the core component of RNP complex involved in localizing mRNAs [45,46], to ensure proper localization of the RNP complex [45][46][47]. Interestingly, our study showed that exuperantia interacted with BmKinesin-1, a Kinesin-1 member, suggesting that in PSG cells, the exuperantia-associated RNP complex could be transported by KLP BmKinesin-1.…”