Many long noncoding RNAs (lncRNAs) are unstable and rapidly degraded in the nucleus by the nuclear exosome. An exosome adaptor complex called NEXT (nuclear exosome targeting) functions to facilitate turnover of some of these lncRNAs. Here we show that knockdown of one NEXT subunit, Mtr4, but neither of the other two subunits, resulted in accumulation of two types of lncRNAs: prematurely terminated RNAs (ptRNAs) and upstream antisense RNAs (uaRNAs). This suggested a NEXT-independent Mtr4 function, and, consistent with this, we isolated a distinct complex containing Mtr4 and the zinc finger protein ZFC3H1. Strikingly, knockdown of either protein not only increased pt/uaRNA levels but also led to their accumulation in the cytoplasm. Furthermore, all pt/uaRNAs examined associated with active ribosomes, but, paradoxically, this correlated with a global reduction in heavy polysomes and overall repression of translation. Our findings highlight a critical role for Mtr4/ZFC3H1 in nuclear surveillance of naturally unstable lncRNAs to prevent their accumulation, transport to the cytoplasm, and resultant disruption of protein synthesis.

Tob is a member of the anti-proliferative protein family, which functions in transcription and mRNA decay. We have previously demonstrated that Tob is involved in the general mechanism of mRNA decay by mediating mRNA deadenylation through interaction with Caf1 and a general RNA-binding protein, PABPC1. Here, we focus on the role of Tob in the regulation of specific mRNA. We show that Tob binds directly to a sequence-specific RNA-binding protein, cytoplasmic polyadenylation element-binding protein 3 (CPEB3). CPEB3 negatively regulates the expression of a target by accelerating deadenylation and decay of its mRNA, which it achieves by tethering to the mRNA. The carboxyl-terminal RNA-binding domain of CPEB3 binds to the carboxyl-terminal unstructured region of Tob. Tob then binds Caf1 deadenylase and recruits it to CPEB3 to form a ternary complex. The CPEB3-accelerated deadenylation was abrogated by a dominant-negative mutant of either Caf1 or Tob. Together, these results indicate that Tob mediates the recruitment of Caf1 to the target of CPEB3 and elicits deadenylation and decay of the mRNA. Our results provide an explanation of how Tob regulates specific biological processes.

Polyadenylation of mRNA precursors is mediated by a large multisubunit protein complex. Here we show that RBBP6 (retinoblastoma-binding protein 6), identified initially as an Rb-and p53-binding protein, is a component of this complex and functions in 39 processing in vitro and in vivo. RBBP6 associates with other core factors, and this interaction is mediated by an unusual ubiquitin-like domain, DWNN (''domain with no name''), that is required for 39 processing activity. The DWNN is also expressed, via alternative RNA processing, as a small single-domain protein (isoform 3 [iso3]). Importantly, we show that iso3, known to be down-regulated in several cancers, competes with RBBP6 for binding to the core machinery, thereby inhibiting 39 processing. Genome-wide analyses following RBBP6 knockdown revealed decreased transcript levels, especially of mRNAs with AU-rich 39 untranslated regions (UTRs) such as c-Fos and c-Jun, and increased usage of distal poly(A) sites. Our results implicate RBBP6 and iso3 as novel regulators of 39 processing, especially of RNAs with AU-rich 39 UTRs.

The nuclear RNA exosome is an essential and versatile machinery that regulates maturation and degradation of a huge plethora of RNA species. The past two decades have witnessed remarkable progress in understanding the whole picture of its RNA substrates and the structural basis of its functions. In addition to the exosome itself, recent studies focusing on associated co-factors have been elucidating how the exosome is directed towards specific substrates. Moreover, it has been gradually realized that loss-of-function of exosome subunits affect multiple biological processes such as the DNA damage response, R-loop resolution, maintenance of genome integrity, RNA export, translation and cell differentiation. In this review, we summarize the current knowledge of the mechanisms of nuclear exosome-mediated RNA metabolism and discuss their physiological significance.

The regulation of mRNA deadenylation constitutes a pivotal mechanism of the post-transcriptional control of gene expression. Here we show that the antiproliferative protein Tob, a component of the Caf1-Ccr4 deadenylase complex, is involved in regulating the expression of the proto-oncogene c-myc. The c-myc mRNA contains cis elements (CPEs) in its 3'-untranslated region (3'-UTR), which are recognized by the cytoplasmic polyadenylation element-binding protein (CPEB). CPEB recruits Caf1 deadenylase through interaction with Tob to form a ternary complex, CPEB-Tob-Caf1, and negatively regulates the expression of c-myc by accelerating the deadenylation and decay of its mRNA. In quiescent cells, c-myc mRNA is destabilized by the trans-acting complex (CPEB-Tob-Caf1), while in cells stimulated by the serum, both Tob and Caf1 are released from CPEB, and c-Myc expression is induced early after stimulation by the stabilization of its mRNA as an 'immediate-early gene'. Collectively, these results indicate that Tob is a key factor in the regulation of c-myc gene expression, which is essential for cell growth. Thus, Tob appears to function in the control of cell growth at least, in part, by regulating the expression of c-myc.