Increased expression of the transcription/translation regulatory protein Y-box binding protein-1 (YB-1) is associated with cancer aggressiveness, particularly in breast carcinoma. Here we establish that YB-1 levels are elevated in invasive breast cancer cells and correlate with reduced expression of E-cadherin and poor patient survival. Enforced expression of YB-1 in noninvasive breast epithelial cells induced an epithelial-mesenchymal transition (EMT) accompanied by enhanced metastatic potential and reduced proliferation rates. YB-1 directly activates cap-independent translation of messenger RNAs encoding Snail1 and other transcription factors implicated in downregulation of epithelial and growth-related genes and activation of mesenchymal genes. Hence, translational regulation by YB-1 is a restriction point enabling coordinated expression of a network of EMT-inducing transcription factors, likely acting together to promote metastatic spread.

YB-1 is a broad-specificity RNA-binding protein that is involved in regulation of mRNA transcription, splicing, translation, and stability. In both germinal and somatic cells, YB-1 and related proteins are major components of translationally inactive messenger ribonucleoprotein particles (mRNPs) and are mainly responsible for storage of mRNAs in a silent state. However, mechanisms regulating the repressor activity of YB-1 are not well understood. Here we demonstrate that association of YB-1 with the capped 5 terminus of the mRNA is regulated via phosphorylation by the serine/threonine protein kinase Akt. In contrast to its nonphosphorylated form, phosphorylated YB-1 fails to inhibit cap-dependent but not internal ribosome entry site-dependent translation of a reporter mRNA in vitro. We also show that similar to YB-1, Akt is associated with inactive mRNPs and that activated Akt may relieve translational repression of the YB-1-bound mRNAs. Using Affymetrix microarrays, we found that many of the YB-1-associated messages encode stress-and growth-related proteins, raising the intriguing possibility that Akt-mediated YB-1 phosphorylation could, in part, increase production of proteins regulating cell proliferation, oncogenic transformation, and stress response.More than two decades ago, several abundant proteins within the size range of ϳ50 to 60 kDa were identified in complexes with maternal mRNA in Xenopus laevis oocytes and reported to be involved in translational "masking" of mRNA during early metazoan development (9, 34). Subsequently, these proteins initially cloned as FRGY1 and FRGY2 (frog Y-box proteins 1 and 2) (42) turned out to be common for male and female germ cells in all organisms studied, including mammals (27, 37). In somatic mammalian cells, the closely related 50-kDa protein (Ͼ96% amino acid identity), first designated p50 and most recently YB-1 (Y-box-binding protein 1), was shown to be a predominant component of translationally inactive messenger ribonucleoprotein particles (mRNPs) (14, 28). Interestingly, YB-1 was independently cloned as a transcription factor that specifically binds to the Y-box promoter element of major histocompatibility complex class II genes (11). It is now well established that YB-1 and related proteins are involved in regulation of both transcription and translation by virtue of sequence-specific and nonspecific binding to nucleic acids (45). The DNA and RNA sequence specificity of YB-1 is mediated through an evolutionarily conserved cold shock domain (CSD), which contains the RNA-binding motifs RNP1 and RNP2. The C terminus of YB-1 possesses alternating basic and acidic clusters and is implicated in both nonspecific DNA or RNA binding and protein-protein interactions (12,24). YB-1 functions as a structural protein involved in spatial organization of mRNPs (36). It is also known to bind in close proximity to the mRNA cap structure and to displace the initiation factors eukaryotic translation initiation factor 4E (eIF4E) and eIF4G, thereby causing mRNA translational silenci...

The Y-box binding protein 1 (YB-1, YBX1) is a member of the family of DNA- and RNA-binding proteins with an evolutionarily ancient and conserved cold shock domain. It falls into a group of intrinsically disordered proteins that do not follow the classical rule 'one protein-one function' but introduce a novel principle stating that a disordered structure suggests many functions. YB-1 participates in a wide variety of DNA/RNA-dependent events, including DNA reparation, pre-mRNA transcription and splicing, mRNA packaging, and regulation of mRNA stability and translation. At the cell level, the multiple activities of YB-1 are manifested as its involvement in cell proliferation and differentiation, stress response, and malignant cell transformation. WIREs RNA 2014, 5:95-110. doi: 10.1002/wrna.1200 CONFLICT OF INTEREST: The authors have declared no conflicts of interest for this article. For further resources related to this article, please visit the WIREs website.

mRNA silencing and storage play an important role in gene expression under diverse circumstances, such as throughout early metazoan development and in response to many types of environmental stress. Here we demonstrate that the major mRNA-associated protein YB-1, also termed p50, is a potent cap-dependent mRNA stabilizer. YB-1 addition or overexpression dramatically increases mRNA stability in vitro and in vivo, whereas YB-1 depletion results in accelerated mRNA decay. The cold shock domain of YB-1 is responsible for the mRNA stabilizing activity, and a blocked mRNA 5¢ end is required for YB-1-mediated stabilization. Signi®cantly, exogenously added YB-1 destabilizes the interaction of the cap binding protein, eIF4E, with the mRNA cap structure. Conversely, sequestration of eIF4E from the cap increases the association of endogenous YB-1 with mRNA at or near the cap, and signi®cantly enhances mRNA stability. These data support a model whereby down-regulation of eIF4E activity or increasing the YB-1 mRNA binding activity or concentration in cells activates a general default pathway for mRNA stabilization.

In eukaryotic cells, the movement of macromolecules between the nucleus and cytoplasm occurs through the nuclear pore complex (NPC)--a large protein complex spanning the nuclear envelope. The nuclear transport of proteins is usually mediated by a family of transport receptors known as karyopherins. Karyopherins bind to their cargoes via recognition of nuclear localization signal (NLS) for nuclear import or nuclear export signal (NES) for export to form a transport complex. Its transport through NPC is facilitated by transient interactions between the karyopherins and NPC components. The interactions of karyopherins with their cargoes are regulated by GTPase Ran. In the current review, we describe the NPC structure, NLS, and NES, as well as the model of classic Ran-dependent transport, with special emphasis on existing alternative mechanisms; we also propose a classification of the basic mechanisms of protein transport regulation.

YB-1 is a DNA/RNA-binding nucleocytoplasmic shuttling protein whose regulatory effect on many DNA-and RNAdependent events is determined by its localization in the cell. Distribution of YB-1 between the nucleus and the cytoplasm is known to be dependent on nuclear targeting and cytoplasmic retention signals located within the C-terminal portion of YB-1. Here, we report that YB-1 undergoes a specific proteolytic cleavage by the 20S proteasome, which splits off the C-terminal 105-amino-acidlong YB-1 fragment containing a cytoplasmic retention signal. Cleavage of YB-1 by the 20S proteasome in vitro appears to be ubiquitin-and ATP-independent, and is abolished by the association of YB-1 with messenger RNA. We also found that genotoxic stress triggers a proteasome-mediated cleavage of YB-1 in vivo and leads to accumulation of the truncated protein in nuclei of stressed cells. Endoproteolytic activity of the proteasome may therefore play an important role in regulating YB-1 functioning, especially under certain stress conditions.

A cDNA encoding the major core protein, p50, of cytoplasmic messenger ribonucleoprotein particles (mRNPs) of somatic cells was cloned from a rabbit reticulocyte cDNA library. From the derived 324-amino acid sequence, p50 is identified as a member of the Y-box binding transcription factor family. The protein was earlier described as a repressor of globin mRNA translation. These findings suggest that p50 may affect protein biosynthesis at two levels: mRNA transcription in the nucleus and mRNA translation in the cytoplasm. Together with recently published results showing that masked mRNA in germ cells also is associated with proteins of the Y-box binding protein family, the present finding indicates that these proteins are universal core proteins responsible for the formation of cytoplasmic mRNPs in eukaryotes. Highly purified p50 forms large 18 S homomultimeric complexes with a molecular mass of about 800 kilodaltons and melts RNA secondary structure. This suggests that p50 may affect translation by changing the overall structure of the mRNA.

The cytoplasmic messenger ribonucleoprotein particles of mammalian somatic cells contain the protein YB-1, also called p50, as a major core component. YB-1 is multifunctional and involved in regulation of mRNA transcription and translation. Our previous studies demonstrated that YB-1 stimulates initiation of translation in vitro at a low YB-1/mRNA ratio, whereas an increase of YB-1 bound to mRNA resulted in inhibition of protein synthesis in vitro and in vivo. Here we show that YB-1-mediated translation inhibition in a rabbit reticulocyte cell-free system is followed by a decay of polysomes, which is not a result of mRNA degradation or its functional inactivation. The inhibition does not change the ribosome transit time, and therefore, it affects neither elongation nor termination of polypeptide chains and only occurs at the stage of initiation. YB-1 induces accumulation of mRNA in the form of free messenger ribonucleoprotein particles, i.e. it blocks mRNA association with the small ribosomal subunit. The accumulation is accompanied by eukaryotic initiation factor eIF4G dissociation from mRNA. The C-terminal domain of YB-1 is responsible for inhibition of translation as well as the disruption of mRNA interaction with eIF4G.All mRNAs in eukaryotic cells are associated with proteins and form messenger ribonucleoprotein particles (mRNPs) 1 (1-6). Some mRNA-associated proteins exhibit specificity for certain mRNA(s); others are universal. To date, two universal major proteins of cytoplasmic mRNPs tightly bound to mRNA are well characterized. One of them is poly(A)-binding protein that stabilizes mRNA (7, 8) and promotes protein synthesis at the initiation stage (9, 10). It is suggested that the protein biosynthesis promotion occurs due to binding of multimerized poly(A)-binding protein associated with poly(A) to eIF4G and 1) mRNA cyclization and facilitation of ribosomal recycling (11, 12) and/or 2) a conformational change in eIF4F and an increase in affinity of eIF4E for the 5Ј-cap (13).The other common mRNP component is YB-1, a 36-kDa protein with abnormal mobility in SDS gel electrophoresis that is typical for this 50-kDa protein (14 -16). According to its primary structure, YB-1 from rabbit reticulocytes was identified as a member of the Y-box (YB) transcription factor family: it was virtually identical (ϳ98% identity) to human YB-1. YB-1 has been shown to participate in different steps of mRNA biogenesis, including mRNA transcription, processing, and transport from the nucleus into the cytoplasm (5, 17, 18) where it can regulate mRNA localization, translation, and mRNA stability (15,16,19). YB-1 displays DNA and RNA melting, annealing, and strand exchange activities, which probably underlies many functions of the YB protein family (20,21).The characteristic feature of these proteins is a central, highly conserved, cold shock domain (CSD) that consists of 80 amino acid residues and exhibits 43% identity to the major Escherichia coli cold shock protein CspA (4, 5). CSD comprises a five-stranded ␤-barrel with RNP ...