The signal sequence coding region promotes nuclear export of mRNA.
In eukaryotic cells, most mRNAs are exported from the nucleus by the transcription export (TREX) complex, which is loaded onto mRNAs after their splicing and capping. We have studied in mammalian cells the nuclear export of mRNAs that code for secretory proteins, which are targeted to the endoplasmic reticulum membrane by hydrophobic signal sequences. The mRNAs were injected into the nucleus or synthesized from injected or transfected DNA, and their export was followed by fluorescent in situ hybridization. We made the surprising observation that the signal sequence coding region (SSCR) can serve as a nuclear export signal of an mRNA that lacks an intron or functional cap. Even the export of an intron-containing natural mRNA was enhanced by its SSCR. Like conventional export, the SSCR-dependent pathway required the factor TAP, but depletion of the TREX components had only moderate effects. The SSCR export signal appears to be characterized in vertebrates by a low content of adenines, as demonstrated by genome-wide sequence analysis and by the inhibitory effect of silent adenine mutations in SSCRs. The discovery of an SSCR-mediated pathway explains the previously noted amino acid bias in signal sequences and suggests a link between nuclear export and membrane targeting of mRNAs.
Department of Cell Biology, Harvard Medical School, Boston, Massachusetts, United States of America.
Alexander F
AF
Palazzo
Michael
M
Springer
Yoko
Y
Shibata
Chung-Sheng
CS
Lee
Anusha P
AP
Dias
Tom A
TA
Rapoport
D000225:Adenine; D000818:Animals; D002460:Cell Line; D002467:Cell Nucleus / Q000235:genetics / Q000378:metabolism*; D002522:Chlorocebus aethiops; D051717:Fushi Tarazu Transcription Factors / Q000235:genetics / Q000378:metabolism; D016678:Genome / Q000235:genetics; D006801:Humans; D051379:Mice; D009865:Oocytes; D016366:Open Reading Frames / Q000235:genetics*; D014176:Protein Biosynthesis; D021382:Protein Sorting Signals / Q000235:genetics*; D012326:RNA Splicing / Q000235:genetics; D034443:RNA Transport; D012333:RNA, Messenger / Q000235:genetics* / Q000378:metabolism*; D038621:Regulatory Sequences, Ribonucleic Acid / Q000235:genetics*; D014981:Xenopus
D051717:Fushi Tarazu Transcription Factors; D021382:Protein Sorting Signals; D012333:RNA, Messenger; D038621:Regulatory Sequences, Ribonucleic Acid; D000225:Adenine