How and why are eukaryotic mRNAs transported and localized to discrete regions of the cell?

Eukaryotic mRNAs are transcribed in the cell nucleus and then exported to the cytoplasm where they are translated into proteins. The transport and localization of mRNAs is essential for proper gene expression, cell differentiation, and developmental processes. In this step-by-step guide, we will examine the molecular mechanisms involved in mRNA transport and localization, and how they contribute to cellular function.

After transcription, eukaryotic mRNAs are subject to various processing events like capping, splicing, and polyadenylation. These modifications help in mRNA stability and facilitate efficient export from the nucleus to the cytoplasm through nuclear pore complexes. Export proteins, such as TAP (Tip-associated protein) and the export receptor NXT1, recognize specific mRNA signals (like the exon-exon junctions) and allow passage through the nuclear pore complexes.

Once in the cytoplasm, the mRNA molecules can be transported to specific regions within the cell through two main processes: active transport and passive diffusion. Active transport is mediated by molecular motors, like dynein and kinesin, which move mRNAs along microtubules, while passive diffusion is facilitated by random molecular movement. In both cases, the mRNAs are bound to RNA-binding proteins, forming ribonucleoprotein (RNP) granules.

RNA-binding proteins (RBPs) play a crucial role in mRNA localization by recognizing and binding to specific mRNA localization elements, called zipcodes. These proteins not only stabilize the mRNAs but also interact with molecular motors (dynein and kinesin) and anchor them to specific locations within the cell. RBPs also regulate mRNA translation, ensuring that the mRNA is not translated until it reaches its proper destination.

mRNA localization is essential for cellular function and development. Localizing mRNAs and their subsequent protein products provides spatial and temporal control over gene expression, which enables the cell to accomplish complex tasks like cell division, differentiation, and migration. Additionally, mRNA localization helps in forming cellular compartments and organizing polarized cells, as seen in neurons, where mRNAs are transported to the dendrites, and oocytes, where mRNA localization establishes the body plan of the developing embryo. In summary, eukaryotic mRNAs are transported and localized to discrete regions of the cell through processes involving nuclear export proteins, RNA binding proteins, and molecular motors. Localization of mRNAs contributes to proper gene expression, cell differentiation, and developmental processes, making RNA localization an essential aspect of cell biology.