11.4
Regulated mRNA Transport
In eukaryotes, transcription and translation are compartmentalized; an mRNA is first synthesized in the nucleus and then selectively transported to th…
The transport of mRNAs from the nucleus to specific locations in the cytoplasm is a highly regulated process aided by a variety of cis- and trans-acting elements.
mRNA contains cis-acting elements at the 3’ and, less often, at the 5’ untranslated regions of the mRNA. These localization elements, also known as zip-code regions, can be up to a thousand base pairs in length and determine the cytoplasmic localization of the exported mRNA.
The trans-acting elements include mRNA binding proteins that recognize specific sequences or structures formed by the mRNA and, together with RNA, form heterogeneous nuclear ribonucleoprotein particles.
mRNA in these particles is then exported through nuclear pore complexes with the help of exporter proteins that associate with both the mRNA and the nuclear pore complexes.
Nucleus-specific proteins detach from the mRNA before it is exported to the cytoplasm while exporter proteins leave the mRNA in the cytoplasm and return to the nucleus for further mRNA export.
Some mRNAs are transported to a specific location as directed by their zip-code sequences before starting protein production and remain translationally inactive until they reach their target location.
The mRNAs are transported to the target location by random diffusion or with the help of cytoskeletal filaments. The mRNAs are then trapped by anchor proteins, which help them to remain at a specified location.
Free ribosomes bind to mRNA and start the process of the translation to produce proteins. If the final protein carries a target sequence for an organelle, it will be directed to the respective organelle.
If the synthesized protein is a cell surface or secreted protein, its target peptide sequence will be recognized, and the whole complex of the mRNA, ribosome, and newly synthesized peptide will be transferred to the surface of the endoplasmic reticulum for further synthesis.
View the full transcript and gain access to JoVE Core videos
Q1: What are zip-code regions and how do they control mRNA localization?
Zip-code regions are cis-acting elements located in the 3' untranslated regions of mRNA, typically up to 1,000 base pairs long. These sequences determine where exported mRNA localizes in the cytoplasm before translation begins. By directing mRNA to specific cellular locations, zip-code regions enable cells to produce high concentrations of proteins near their target destinations without requiring additional protein transport.
Q2: How do mRNA binding proteins and nuclear pore complexes work together during mRNA export?
mRNA binding proteins recognize specific sequences or structures in mRNA and form heterogeneous nuclear ribonucleoprotein particles with the RNA. These particles are then transported through nuclear pore complexes with assistance from exporter proteins that bind both the mRNA and the pore complex. Nucleus-specific proteins detach before export, while exporter proteins leave the mRNA in the cytoplasm and return to the nucleus for additional mRNA export cycles.
Q3: What happens to mRNA after it reaches its target location in the cytoplasm?
Once mRNA reaches its target location, anchor proteins trap it in place, keeping it localized and translationally inactive until the appropriate signal. Free ribosomes then bind to the mRNA and initiate translation to produce proteins. If the synthesized protein carries an organelle-targeting sequence, it is directed to the respective organelle; if it is a secretory or cell surface protein, the mRNA-ribosome complex transfers to the endoplasmic reticulum for continued synthesis.
Q4: How does mRNA transport to the cytoplasm differ between normal and HIV-infected cells?
Normally, mRNA is exported from the nucleus only after complete post-transcriptional processing including splicing and polyadenylation. However, HIV circumvents this rule by using a protein called Rev, which binds to specific sequences in intron-carrying pre-mRNA and recruits nuclear export receptors. This allows HIV to transport unprocessed pre-mRNA directly from the nucleus to the cytoplasm, enabling viral gene expression before full mRNA maturation occurs.
Q5: What is the advantage of localizing mRNA to specific cytoplasmic sites before translation?
mRNA localization allows cells to produce high concentrations of proteins precisely where they are needed in the cytoplasm. This targeted approach eliminates the need for cells to expend resources transporting proteins to their final destinations after synthesis. By positioning mRNA at specific locations before translation begins, cells achieve efficient, localized protein production that supports specialized cellular functions.
Q6: How do cytoskeletal filaments contribute to mRNA transport within the cytoplasm?
After mRNA is exported to the cytoplasm, it travels to its target location either through random diffusion or with directed assistance from cytoskeletal filaments. Cytoskeletal filaments provide organized tracks that guide mRNA movement to specific cellular regions. Once the mRNA reaches its destination, anchor proteins secure it in place, ensuring stable localization for subsequent translation and protein synthesis at the correct cellular location.
Q7: What role do cis-acting elements play in regulating mRNA transport and localization?
Cis-acting elements are regulatory sequences within the mRNA itself, primarily located in the 3' and occasionally 5' untranslated regions. These elements serve as recognition sites for trans-acting mRNA binding proteins that facilitate nuclear export and cytoplasmic localization. By encoding specific sequences or structures, cis-acting elements determine which mRNAs are exported, where they localize in the cytoplasm, and when they become available for translation.