Pur␣, which is involved in diverse aspects of cellular functions, is strongly expressed in neuronal cytoplasm. Previously, we have reported that this protein controls BC1 RNA expression and its subsequent distribution within dendrites and that Pur␣ is associated with polyribosomes. Here, we report that, following treatment with EDTA, Pur␣ was released from polyribosomes in mRNA/protein complexes (mRNPs), which also contained mStaufen, Fragile X Mental Retardation Protein (FMRP), myosin Va, and other proteins with unknown functions. As the coimmunoprecipitation of these proteins by an anti-Pur␣ antibody was abolished by RNase treatment, Pur␣ may assist mRNP assembly in an RNAdependent manner and be involved in targeting mRNPs to polyribosomes in cooperation with other RNA-binding proteins. The immunoprecipitation of mStaufenand FMRP-containing mRNPs provided additional evidence that the anti-Pur␣ detected structurally or functionally related mRNA subsets, which are distributed in the somatodendritic compartment. Furthermore, mRNPs appear to reside on rough endoplasmic reticulum equipped with a kinesin motor. Based on our present findings, we propose that this rough endoplasmic reticulum structure may form the molecular machinery that mediates and regulates multistep transport of polyribosomes along microtubules and actin filaments, as well as localized translation in the somatodendritic compartment.

Neural BC1 RNA is distributed in neuronal dendrites as RNA-protein complexes (BC1 RNPs) containing Translin. In this study, we demonstrated that the single-stranded DNA-and RNA-binding protein pur ␣ and its isoform, pur ␤, which have been implicated in control of DNA replication and transcription, linked BC1 RNA to microtubules (MTs). The binding site was within the 5Ј proximal region of BC1 RNA containing putative dendrite-targeting RNA motifs rich in G and U residues, suggesting that in the cytoplasm of neurons, these nuclear factors are involved in the BC1 RNA transport along dendritic MTs. The pur proteins were not components of BC1 RNP but appeared to associate with MTs in brain cells. Therefore, it is suggested that they may transiently interact with the RNP during transport. In this respect, the interaction of pur proteins with BC1 RNA could be regulated by the Translin present within the RNP, because the binding mode of these two classes of proteins (pur proteins and Translin) to the dendrite-targeting RNA motifs was mutually exclusive. As the motifs are well conserved in microtubule-associated protein 2a/b mRNA as well, the pur proteins may also play a role(s) in the dendritic transport of a subset of mRNAs. Key Words: BC1 RNABrain-Neuronal dendrite -Pur ␣ and pur ␤ proteinsMicrotubule-binding protein-RNA transport. J. Neurochem. 75, 1781Neurochem. 75, -1790Neurochem. 75, (2000.Neural BC1 RNA is expressed selectively in rodent brain (Sutcliffe et al., 1982;Anzai et al., 1986) and is distributed in neuronal dendrites (Tiedge et al., 1991) in the form of ribonucleoprotein particles (RNPs) (Kobayashi et al., 1991;Cheng et al., 1996). In a previous study, we demonstrated the presence of two copies of y-, h-element homologous sequences in BC1 RNA [nucleotides (nt) 1-22 and 47-76] (see Fig. 1) (Muramatsu et al., 1998), and we detected two RNA-binding proteins using the radiolabeled 3Ј distal y-, h-elements (nt 47-76) as a probe, which co-purified with BC1 RNP (Kobayashi et al., 1998;Muramatsu et al., 1998). One of these proteins was mouse Translin and the other was a 37-kDa protein. In addition to these observations, we demonstrated the dendritic distribution of Translin protein in hippocampal neurons in primary culture (Kobayashi et al., 1998), suggesting that Translin is involved in the dendritic translocation of BC1 RNP. Recently, Translin was also detected in dendrites of a subset of neurons of the mouse brain (Wu et al., 1999). Wu et al. (1997) reported that testis/brain RNA-binding protein (TB-RBP), which was originally identified as a y-, h-element-binding protein (Kwon and Hecht, 1991), is the mouse homologue of Translin. They also reported that TB-RBP has translational repressor activity (Kwon and Hecht, 1993) and links a subset of mRNAs to microtubules (MTs) in vitro through binding to their y-, h-sequence elements (Han et al., 1995a). These observations in turn suggested that dendritic MTs are also involved in the process of BC1 RNP transport and that this RNP may have regulatory roles...

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We have previously reported that pur a a, known to be a regulator of DNA replication and transcription, links neural BC1 RNA to microtubules via dendrite-targeting RNA motifs. Here we demonstrate the subcellular localization of pur proteins within the brain. Pur proteins were detected in neurons but not in glia. Immunohistochemical staining was prominent in perikarya and proximal dendrites and also extended into primary dendritic processes, but no significant signals were detected in the distal regions of dendrite. When homogenates of mouse brain were fractionated, pur a a was most concentrated in the microsomal pellet. Consistently, pur a a co-fractionated with free polysomes as well as with membrane-bound polysomes and the association with polysomes was mediated by binding ribosomal subunits. Levels of ribosomes with pur a a progressively increased during postnatal development of the brain.

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