Conservation of the RNA Transport Machineries and Their Coupling to Translation Control across Eukaryotes
Figure 2
Transport and translation repression of osk mRNA during Drosophila oogenesis. osk mRNA is synthesized in the nucleus (ncn) of the nurse cells (nc) and exported already as a complex with several factors controlling its transport and/or translation (light blue circles), like the exon junction complex (EJC, composed of Mago-Nashi/Y14/eIF4AIII), Hrp48, Bru and Sqd. In the nc cytoplasm more factors controlling translation (Me31B, Cup, Bru, PTB, Imp), localization (Stau, Exu, Sqd, Btz, Pabp) or stability (Pabp) (light blue circles) associate with osk mRNA to form a big RNP complex (light blue circles). This RNP contains many osk mRNA molecules and multiple factors that repress translation of osk by several different mechanisms (see text for details). This big silenced osk RNP is recruited by the Bic-D/Egl/Dyn localization machinery which directs its minus end directed microtubule transport in the nurse cell cytoplasm and through the ring canals into the oocyte (oo). Factors linking osk RNPs to the transport machinery are not known. Since Egl binds directly some other localized mRNAs, Egl may be the linking factor. Other proteins in complex with Bic-D, such as Pabp, (which binds directly to osk mRNA through adenine rich sequences (ARS) and the poly-(A) tail) could also be involved. Within the oocyte the silenced osk RNP is then transported by a kinesin motor probably by a random walk process in a poorly polarized microtubule network with a net movement toward the posterior cortex. This movement is followed by a short-range actomyosin-dependent transport or entrapment of osk mRNA to the posterior cortex. During its journey osk mRNA associates with the factors repressing its translation. Although different proteins may associate with osk during different stages of oogenesis, most of them are probably associated with it during its all trip to the posterior. When osk mRNA reaches the posterior cortex at stage 9 of oogenesis, translation repression is relieve and the mRNA gets translated (not shown in this figure).
