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Sac3 is an mRNA export factor that localizes to cytoplasmic fibrils of nuclear pore complex
Elissa P Lei1, Charlene A Stern, Birthe Fahrenkrog
1Department of Biological Chemistry and Molecular Pharmacology, Harvard Medical School and Dana Farber Cancer Institute, Boston, Massachusetts 02115, USA.
Abstract:
In eukaryotes, mRNAs are transcribed in the nucleus and exported to the cytoplasm for translation to occur. Messenger RNAs complexed with proteins referred to as ribonucleoparticles are recognized for nuclear export in part by association with Mex67, a key Saccharomyces cerevisiae mRNA export factor and homolog of human TAP/NXF1. Mex67, along with its cofactor Mtr2, is thought to promote ribonucleoparticle translocation by interacting directly with components of the nuclear pore complex (NPC). Herein, we show that the nuclear pore-associated protein Sac3 functions in mRNA export. Using a mutant allele of MTR2 as a starting point, we have identified a mutation in SAC3 in a screen for synthetic lethal interactors. Loss of function of SAC3 causes a strong nuclear accumulation of mRNA and synthetic lethality with a number of mRNA export mutants. Furthermore, Sac3 can be coimmunoprecipitated with Mex67, Mtr2, and other factors involved in mRNA export. Immunoelectron microscopy analysis shows that Sac3 localizes exclusively to cytoplasmic fibrils of the NPC. Finally, Mex67 accumulates at the nuclear rim when SAC3 is mutated, suggesting that Sac3 functions in Mex67 translocation through the NPC.
Insights
The nuclear pore protein Sac3 is crucial for mRNA export in yeast. Sac3 facilitates the translocation of mRNA export factor Mex67 through the nuclear pore complex (NPC).
Area of Science:
- Molecular Biology
- Cell Biology
- Genetics
Background:
- Messenger RNA (mRNA) export from the nucleus to the cytoplasm is essential for gene expression in eukaryotes.
- Mex67, a key mRNA export factor in Saccharomyces cerevisiae, works with its cofactor Mtr2 to facilitate mRNA-protein complex translocation through the nuclear pore complex (NPC).
Purpose of the Study:
- To investigate the role of the nuclear pore-associated protein Sac3 in mRNA export.
- To identify novel factors involved in the mRNA export pathway.
Main Methods:
- Genetic screening for synthetic lethal interactors with a mutant allele of MTR2.
- Co-immunoprecipitation assays to assess protein interactions.
- Immunoelectron microscopy to determine Sac3 localization.
- Analysis of mRNA export efficiency in wild-type and mutant strains.
Main Results:
- A mutation in SAC3 was identified as a synthetic lethal interactor with a mutation in MTR2.
- Loss of Sac3 function leads to significant nuclear mRNA accumulation and synthetic lethality with other mRNA export mutants.
- Sac3 co-immunoprecipitated with Mex67, Mtr2, and other mRNA export factors.
- Sac3 localizes to the cytoplasmic fibrils of the NPC.
- Mutation of SAC3 caused Mex67 to accumulate at the nuclear rim, indicating a role in Mex67 translocation.
Conclusions:
- Sac3 is a novel and essential component of the mRNA export pathway in yeast.
- Sac3 functions in the translocation of the Mex67-Mtr2 complex through the NPC.
- Sac3's localization and interaction data suggest a role in facilitating mRNA export by mediating Mex67 passage through the nuclear pore.