The signal sequence coding region promotes nuclear export of mRNA

Area of Science:

• Molecular Biology
• Cell Biology
• Genetics

Background:

• Most eukaryotic mRNAs are exported from the nucleus via the transcription export (TREX) complex.
• Secretory proteins are targeted to the endoplasmic reticulum membrane by hydrophobic signal sequences.
• The mechanism of nuclear export for mRNAs encoding secretory proteins is not fully understood.

Purpose of the Study:

• To investigate the nuclear export pathway of mRNAs encoding secretory proteins in mammalian cells.
• To determine if the signal sequence coding region (SSCR) influences mRNA nuclear export.
• To elucidate the characteristics and requirements of the SSCR-mediated export pathway.

Main Methods:

• Injection of mRNAs into the nucleus or synthesis from injected/transfected DNA.
• Fluorescent in situ hybridization to track mRNA export.
• Genome-wide sequence analysis and mutation studies to identify sequence characteristics.
• Depletion of TREX components and TAP factor to assess pathway requirements.

Main Results:

• The SSCR can function as a nuclear export signal for mRNAs lacking introns or functional caps.
• SSCR enhanced the export of intron-containing natural mRNAs.
• The SSCR-dependent pathway requires the factor TAP, with moderate effects observed upon TREX component depletion.
• Vertebrate SSCRs characterized by low adenine content; silent adenine mutations inhibited export.

Conclusions:

• A novel SSCR-mediated mRNA nuclear export pathway exists, independent of canonical TREX complex function.
• This pathway explains amino acid bias in signal sequences and links mRNA export to membrane targeting.
• The SSCR acts as a functional nuclear export signal, particularly in vertebrates, with specific sequence requirements.