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ZBP1-modulated localization of β-actin mRNA enables a cell to establish polarity and structural asymmetry. Although the mechanism of β-actin mRNA localization has been well established, the underlying mechanism of how a specific molecular motor contributes to the transport of the ZBP1 (also known as IGF2BP1) complex in non-neuronal cells remains elusive. In this study, we report the isolation and identification of KIF11, a microtubule motor, which physically interacts with ZBP1 and is a component of β-actin messenger ribonucleoprotein particles (mRNPs). We show that KIF11 colocalizes with the β-actin mRNA, and the ability of KIF11 to transport β-actin mRNA is dependent on ZBP1. We characterize the corresponding regions of ZBP1 and KIF11 that mediate the interaction of the two proteins in vitro and in vivo. Disruption of the in vivo interaction of KIF11 with ZBP1 delocalizes β-actin mRNA and affects cell migration. Our study reveals a molecular mechanism by which a particular microtubule motor mediates the transport of an mRNP through direct interaction with an mRNA-binding protein. [ABSTRACT FROM AUTHOR]

Reports on the transcription of messenger RNA (mRNA) molecules in the nucleus before they undergo export into the cytoplasm, where they are translated to produce proteins. Features of mRNA localization systems.

The actin-related protein 2⁄3 (Arp2⁄3) complex is a crucial actin polymerization nucleator and is localized to the leading protrusions of migrating cells. However, how the multiprotein complex is targeted to the protrusions remains unknown. Here, we demonstrate that mRNAs for the seven subunits of the Arp2⁄3 complex are localized to the protrusions in fibroblasts, supporting a hypothesis that the Arp2⁄3 complex is targeted to its site of function by mRNA localization. Depletion of serum from culture medium inhibits Arp2⁄3-complex mRNA localization to the protrusion, whereas serum stimulation leads to significant mRNA localization within 30 minutes. The effect of serum suggests that Arp2⁄3-complex mRNA localization is a cellular response to extracellular stimuli. The localization of the Arp2⁄3 complex mRNAs is dependent on both actin filaments and microtubules, because disruption of either cytoskeletal system (with cytochalasin D and colchicine, respectively) inhibited the localization of all seven subunit mRNAs. In addition, myosin inhibitors significantly inhibit Arp2 mRNA localization in chicken embryo fibroblasts, suggesting a myosin motor dependent mechanism for Arp2⁄3-complex mRNA localization. [ABSTRACT FROM AUTHOR]

Nuclear transcribed genes produce mRNA transcripts destined to travel from the site of transcription to the cytoplasm for protein translation. Certain transcripts can be further localized to specific cytoplasmic regions. We examined the life cycle of a transcribed β-actin mRNA throughout gene expression and localization, in a cell system that allows the in vivo detection of the gene locus, the transcribed mRNAs and the cytoplasmic β-actin protein that integrates into the actin cytoskeleton. Quantification showed that RNA polymerase II elongation progressed at a rate of 3.3 kb/minute and that transactivator binding to the promoter was transient (40 seconds), and demonstrated the unique spatial structure of the coding and non-coding regions of the integrated gene within the transcription site. The rates of gene induction were measured during interphase and after mitosis, demonstrating that daughter cells were not synchronized in respect to transcription initiation of the studied gene. Comparison of the spatial and temporal kinetics of nucleoplasmic and cytoplasmic mRNA transport showed that the ?-actin-localization response initiates from the existing cytoplasmic mRNA pool and not from the newly synthesized transcripts arising after gene induction. It was also demonstrated that mechanisms of random movement were predominant in mediating the efficient translocation of mRNA in the eukaryotic cell. [ABSTRACT FROM AUTHOR]

ZBP1 (zipcode-binding protein 1, also known as IMP-1) is an mRNA regulator, functioning in mRNA localization, stability and translational control. ZBP1 is actively expressed during embryogenesis and tumorigenesis, but its expression is repressed in metastatic breast-cancer cell lines and tumors. In this article, we show that downregulation of ZBP1 expression results from its promoter methylation, an epigenetic process that remodels the chromatin structure and frequently represses gene activity. Demethylation of the ZBP1 promoter in metastatic cells reactivated ZBP1 expression, owing to restoration of the interaction of the ZBP1 promoter with β-catenin. Loss of ZBP1 function not only increased growth ability of metastatic cells, but also promoted cell migration. We identified a number of mRNAs that were selectively associated with ZBP1 in breast-cancer cells. Many of these are involved in cell motility and in cell-cycle regulation, and displayed altered expression patterns in the absence of ZBP1. These data suggest that repression of ZBP1 deregulates its associated mRNAs, leading to the phenotypic changes of breast cancers. [ABSTRACT FROM AUTHOR]