Polypyrimidine tract-binding protein inhibits translation of bip mRNA

Nucleocytoplasmic shuttling of polypyrimidine tract-binding protein is uncoupled from RNA export

Polypyrimidine tract binding protein, PTB/hnRNP I, is involved in pre-mRNA processing in the nucleus and RNA localization and translation in the cytoplasm. In this report, we demonstrate that PTB shuttles between the nucleus and cytoplasm in an energy-dependent manner. Deletion mutagenesis demonstrated that a minimum of the N terminus and RNA recognition motifs (RRMs) 1 and 2 are necessary for nucleocytoplasmic shuttling. Deletion of RRM3 and 4, domains that are primarily responsible for RNA binding, accelerated the nucleocytoplasmic shuttling of PTB. Inhibition of transcription directed by either RNA polymerase II alone or all RNA polymerases yielded similar results. In contrast, selective inhibition of RNA polymerase I did not influence the shuttling kinetics of PTB. Furthermore, the intranuclear mobility of GFP-PTB, as measured by fluorescence recovery after photobleaching analyses, increased significantly in transcriptionally inactive cells compared with transcriptionally active cells. These observations demonstrate that nuclear RNA transcription and export are not necessary for the shuttling of PTB. In addition, binding to nascent RNAs transcribed by RNA polymerase II and/or III retards both the nuclear export and nucleoplasmic movement of PTB. The uncoupling of PTB shuttling and RNA export suggests that the nucleocytoplasmic shuttling of PTB may also play a regulatory role for its functions in the nucleus and cytoplasm.

The 5'-untranslated region of the FMR1 message facilitates translation by internal ribosome entry

Fragile X syndrome, the leading heritable form of mental impairment, is generally caused by large expansions of a CGG repeat in the promoter region of the FMR1 gene followed by transcriptional silencing. However, there is growing evidence that translation of the FMR1 message is also impaired, presumably because of the expanded CGG element in the 5'-untranslated region (5'-UTR) of the FMR1 message. To study the properties of the FMR1 5'-UTR, deletions were generated within a normal 5'-UTR with 16 CGG repeats for both monocistronic and dicistronic (luciferase) reporter constructs. Transient transfection experiments revealed a approximately 20-nucleotide region upstream of the CGG repeat element that functions as an internal ribosome entry site (IRES). The normal CGG element itself does not appear to influence the efficiency of IRES-mediated stimulation of downstream reporter activity (approximately 18-fold over controls). Additional controls indicate that the enhanced activity of the downstream reporter is not due to readthrough from the upstream cistron, nor is it due to translation of cryptic monocistronic transcripts. The role of the FMR1 IRES element is not known at present; however, by analogy to other IRES-containing mRNAs expressed in neurons, the FMR1 IRES element may help to promote translation in dendrites.

Translational Control of the Picornavirus Phenotype

Picornaviruses are small animal viruses with positive-strand genomic RNA, which is translated using cap-independent internal translation initiation. The key role in this is played by ciselements of the 5"-untranslated region (5"-UTR) and, in particular, by the internal ribosome entry site (IRES). The function of translational ciselements requires both canonical translation initiation factors (eIFs) and additional IRES trans-acting factors (ITAFs). All known ITAFs are cell RNA-binding proteins which play a variety of functions in noninfected cells. Specific features of translational ciselements substantially affect the phenotype and, in particular, tissue tropism and pathogenic properties of picornaviruses. It is clear that, in some cases, the molecular mechanism involved is a change in interactions between viral ciselements and ITAFs. The properties and tissue distribution of ITAFs may determine the biological properties of other viruses that also use the IRES-dependent translation initiation. Since this mechanism is also involved in translation of several cell mRNAs, ITAF may contribute to the regulation of the most important aspects of the living activity in noninfected cells.