Conservation of the RNA Transport Machineries and Their Coupling to Translation Control across Eukaryotes

Abstract

Restriction of proteins to discrete subcellular regions is a common mechanism to establish cellular asymmetries and depends on a coordinated program of mRNA localization and translation control. Many processes from the budding of a yeast to the establishment of metazoan embryonic axes and the migration of human neurons, depend on this type of cell polarization. How factors controlling transport and translation assemble to regulate at the same time the movement and translation of transported mRNAs, and whether these mechanisms are conserved across kingdoms is not yet entirely understood. In this review we will focus on some of the best characterized examples of mRNA transport machineries, the "yeast locasome" as an example of RNA transport and translation control in unicellular eukaryotes, and on the Drosophila Bic-D/Egl/Dyn RNA localization machinery as an example of RNA transport in higher eukaryotes. This focus is motivated by the relatively advanced knowledge about the proteins that connect the localizing mRNAs to the transport motors and the many well studied proteins involved in translational control of specific transcripts that are moved by these machineries. We will also discuss whether the core of these RNA transport machineries and factors regulating mRNA localization and translation are conserved across eukaryotes.

Figure 1

Transport and translation repression of ASH1 mRNA in S. cerevisiae. ASH1 mRNA is synthesized in the nucleus of the mother cell. The She2p protein is loaded onto ASH1 mRNA in the nucleus. Once in the cytoplasm, the ASH1-She2p complex binds to She3p which associates with Myo4p to form the transport machinery called the “locasome”. The translation repressors Puf6p and Khd1p and Pabp1 (which is needed for localization) are thought to be also loaded onto ASH1 mRNA before nuclear export. The locasome then transports silenced ASH1 RNPs to the bud through the actin filaments. Puf6p and Khd1p block AHS1 mRNA translation during transport by different mechanisms. One of them is through the interaction of Puf6p with eIF5B and further inhibition of the recruitment of the 60S ribosomal to the mRNA. Khd1p binds eIF4G. This interaction might prevent the recruitment of the 43S pre-initiation complex (consisting of the 40S subunit, the stabilizing factors eIF3, eIF1 and eIF1A and a ternary complex composed of eIF2 bound to an initiator Met-tRNA and GTP) to the mRNA, thereby blocking translation initiation. However, the exact mechanism is not clearly understood. Once the complex is localized to the bud tip, membrane associated kinases, CK2 and Yck1p, phosphorylate Puf6p and Khd1p respectively. This produces the dissociation of the repressors from ASH1 mRNA allowing thus translation activation. Ash1p then inhibits mating-type switching only in the daughter cell.

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).