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Heterogeneous nuclear ribonucleoprotein (hnRNP) A2 is a trans-acting RNA-binding protein that mediates trafficking of RNAs containing the cis-acting A2 response element (A2RE). Previous work has shown that A2RE RNAs are transported to myelin in oligodendrocytes and to dendrites in neurons. hnRNP E1 is an RNA-binding protein that regulates translation of specific mRNAs. Here, we show by yeast two-hybrid analysis, in vivo and in vitro coimmunoprecipitation, in vitro cross-linking, and fluorescence correlation spectroscopy that hnRNP E1 binds to hnRNP A2 and is recruited to A2RE RNA in an hnRNP A2-dependent manner. hnRNP E1 is colocalized with hnRNP A2 and A2RE mRNA in granules in dendrites of oligodendrocytes. Overexpression of hnRNP E1 or microinjection of exogenous hnRNP E1 in neural cells inhibits translation of A2RE mRNA, but not of non-A2RE RNA. Excess hnRNP E1 added to an in vitro translation system reduces translation efficiency of A2RE mRNA, but not of nonA2RE RNA, in an hnRNP A2-dependent manner. These results are consistent with a model where hnRNP E1 recruited to A2RE RNA granules by binding to hnRNP A2 inhibits translation of A2RE RNA during granule transport.

The interaction of E. coli's integration Host Factor (IHF) with fragments of lambda DNA containing the cos site has been studied by gel-mobility retardation and electron microscopy. The cos fragment used in the mobility assays is 398 bp and spans a region from 48,298 to 194 on the lambda chromosome. Several different complexes of IHF with this fragment can be distinguished by their differential mobility on polyacrylamide gels. Relative band intensities indicate that the formation of a complex between IHF and this DNA fragment has an equilibrium binding constant of the same magnitude as DNA fragments containing lambda's attP site. Gel-mobility retardation and electron microscopy have been employed to show that IHF sharply bends DNA near cos and to map the bending site. The protein-induced bend is near an intrinsic bend due to DNA sequence. The position of the bend suggests that IHF's role in lambda DNA packaging may be the enhancement of terminase binding/cos cutting by manipulating DNA structure.

In neural cells, such as oligodendrocytes and neurons, transport of certain RNAs along microtubules is mediated by the cis-acting heterogeneous nuclear ribonucleoprotein A2 response element (A2RE) trafficking element and the cognate trans-acting heterogeneous nuclear ribonucleoprotein (hnRNP) A2 trafficking factor. Using a yeast two-hybrid screen, we have identified a microtubule-associated protein, tumor overexpressed gene (TOG)2, as an hnRNP A2 binding partner. The C-terminal third of TOG2 is sufficient for hnRNP A2 binding. TOG2, the large protein isoform of TOG, is the only isoform detected in oligodendrocytes in culture. TOG coimmunoprecipitates with hnRNP A2 present in the cytoskeleton (CSK) fraction of neural cells, and both coprecipitate with microtubule stabilized pellets. Staining with anti-TOG reveals puncta that are localized in proximity to microtubules, often at the plus ends. TOG is colocalized with hnRNP A2 and A2RE-mRNA in trafficking granules that remain associated with CSK-insoluble tissue. These data suggest that TOG mediates the association of hnRNP A2-positive granules with microtubules during transport and/or localization. INTRODUCTIONThe heterogeneous nuclear ribonucleoprotein (hnRNP) A2 is implicated in RNA processing, RNA transport, and translation regulation (Dreyfuss et al., 1993;Siomi and Dreyfuss, 1995;Hamilton et al., 1999;Kwon et al., 1999). HnRNP A2 binds to a specific cis-acting element, hnRNP A2 response element (A2RE), found in several dendritically localized mRNAs Munro et al., 1999). This interaction is necessary for transport of A2RE-mRNAs to the periphery of neural cells (Munro et al., 1999;Shan et al., 2003).A2RE-mRNAs, hnRNP A2, and other components form complexes that look by light microscopy like granules that are transported to the cell periphery by a microtubule-and kinesin-based mechanism. In cultured oligodendrocytes and neurons, granules containing A2RE-mRNAs and hnRNP A2 are associated with the cytoskeleton (CSK) . HnRNP A2 and some A2RE-mRNAs copurify with the CSK from rat brain (Hoek et al., 1998;Boccaccio et al., 1999).To identify molecular partners of hnRNP A2, we performed yeast two-hybrid analysis. One of the components identified is the tumor overexpressed gene (TOG) protein, a microtubule-associated protein (MAP) ubiquitously expressed in human tissues. TOG is a single copy gene that generates two mRNA splice variants (Nagase et al., 1995;Charrasse et al., 1998) encoding proteins that differ by a 60-aa insert near the C terminus. In dividing cells, TOG localizes with centrosome and spindle microtubules and may be necessary for microtubule rearrangements and spindle assembly (Charrasse et al., 1998;Lee et al., 2001). Orthologues of colonic and hepatic tumor overexpressed gene (ch-TOG) (Homo sapiens), Dis1p, Stu2p, XMAP215, ZYG-9, and mini spindles, in fission yeast, budding yeast, Xenopus laevis, Caenorhabditis elegans, and Drosophila melanogaster, respectively, have similar functions. TOG and its orthologues all contain multiple HEAT repeats that may serv...

SummaryIn oligodendrocytes and neurons genetic information is transmitted from nucleus to dendrites in the form of RNA granules. Here we describe how transport of multiple different RNA molecules in individual granules is analogous to the process of multiplexing in telecommunications. In both cases multiple messages are combined into a composite signal for transmission on a single carrier. Multiplexing provides a mechanism to coordinate local expression of ensembles of genes in myelin in oligodendrocytes and at synapses in neurons.