Role of Polypyrimidine Tract Binding Protein in the Function of the Hepatitis B Virus Posttranscriptional Regulatory Element

Abstract

The hepatitis B virus posttranscriptional regulatory element (PRE) is an RNA element that increases the expression of unspliced mRNAs, apparently by facilitating their export from the nucleus. We have identified a cellular protein that binds to the PRE as the polypyrimidine tract binding protein (PTB), which shuttles rapidly between the nucleus and the cytoplasm. Mutants of the PRE with mutations in PTB binding sites show markedly decreased activity, while cells that stably overexpress PTB show increased PRE-dependent gene expression. Export of PTB from the nucleus, like PRE function, is blocked by a mutant form of Ran binding protein 1 but not by leptomycin B. Therefore, PTB is important for PRE activity and appears to function as an export factor for PRE-containing mRNAs.Eucaryotic mRNA transcription takes place in the nucleus, but translation occurs in the cytoplasm, necessitating the export of mRNA through nuclear pores. In mammalian cells, this export is strictly controlled, in that only fully spliced and processed mRNA is exported (22,27). Part of the control is at the level of retention of incompletely spliced mRNA, probably by splicing factors binding to splice sites. However, this retention mechanism cannot explain all of the available data. First, some genes give rise to alternatively spliced transcripts, in which some of the mature mRNAs still contain splice sites. Second, for at least some cellular genes (e.g., the ␤-globin gene), removal of all introns leads to a defect in the export of mRNA to the cytoplasm (4). Therefore, the presence of splice sites does not always preclude RNA export, while the absence of splice sites does not always lead to export. These data imply that at least some mRNAs contain cis-acting elements that can effect export independently of splicing.In recent years, the existence of such RNA export elements has been confirmed. The best-studied element is the Rev response element (RRE) of human immunodeficiency virus (HIV) and related lentiviruses (6,14). Like almost all retroviruses, HIV contains only one promoter that gives rise to a transcript that is alternatively spliced, resulting in the export of completely spliced, partially spliced, and unspliced mRNAs into the cytoplasm. HIV codes for a trans-acting protein product that modulates the relative amounts of completely spliced versus incompletely spliced and unspliced messages. This protein, called Rev, binds to the RRE in the nucleus and strongly enhances the export of RRE-containing, unspliced or incompletely spliced transcripts. It has become clear that Rev contains a leucine-rich nuclear export signal (NES) that allows it to form a trimolecular complex with two cellular proteins, Crm1 (exportin) and Ran (8,9,29,34). This complex, together with its RNA cargo, interacts with components of the nuclear pore in order to migrate into the cytoplasm. Rev is then stripped off the mRNA in the cytoplasm and is recycled back into the nucleus by virtue of a nuclear localization signal.Other retroviruses also contain RNA export elemen...

Inhibition of Hepatitis B Virus Replication by MyD88 Involves Accelerated Degradation of Pregenomic RNA and Nuclear Retention of Pre-S/S RNAs

“…In a manner similar to that of RBP9-27, MyD88 inhibits PRE-mediated HBV expression by targeting PTB (Fig. 9), an export factor for PRE-containing RNA (54). Interestingly, MyD88 exerted this effect only on HBV-infected cells (Fig.…”

“…The PRE mediates the nuclear export of viral pre-S/S RNAs, but it does not affect the nuclear export of pregenomic RNA (15,(18)(19)(20). In addition, viral pre-S/S RNAs lacking the PRE fail to translocate to the cytoplasm and degrade in the nucleus via a mechanism that has remained elusive (54,55).…”

Polypyrimidine Tract Binding Protein Induces Human Papillomavirus Type 16 Late Gene Expression by Interfering with Splicing Inhibitory Elements at the Major Late 5′ Splice Site, SD3632

“…It encodes a posttranscriptional regulatory element (PRE) that increases the expression of unspliced HBV mRNAs by facilitating their export from the nucleus. The interaction of PTB with the HBV PRE suggested a role for PTB in the nuclear export of these viral mRNAs (31,57). PTB has been shown previously to have a shuttling function, which appears to be Crm1 independent (31,57).…”

Knockdown of polypyrimidine tract-binding protein suppresses ovarian tumor cell growth and invasiveness in vitro

“…It is now widely believed that PTB functions as a negative regulator of pre-mRNA splicing, blocking the inclusion of numerous alternative exons into mRNA (Black, 2003), including its own exon . Besides roles in splicing, PTB has also been implicated in the regulation of other aspects of RNA metabolism, such as premRNA polyadenylation (Castelo-Branco et al, 2004), mRNA stability (Kosinski et al, 2003;Knoch et al, 2004), mRNA export from the nucleus (Zang et al, 2001) and mRNA localization in the cytoplasm (Cote et al, 1999). In addition, PTB is involved in the control of cap-independent translation driven by the internal ribosomal entry site (IRES).…”