Absence of the fragile X mental retardation protein (FMRP) causes fragile X syndrome, the most common form of hereditary mental retardation. FMRP is a mainly cytoplasmic protein thought to be involved in repression of translation, through a complex network of protein-protein and protein-RNA interactions. Most of the currently known protein partners of FMRP recognise the conserved N terminus of the protein. No interaction has yet been mapped to the highly charged, poorly conserved C terminus, so far thought to be involved in RNA recognition through an RGG motif. In the present study, we show that a two-hybrid bait containing residues 419-632 of human FMRP fishes out a protein that spans the sequence of the Ran-binding protein in the microtubule-organising centre (RanBPM/RanBP9). Specific interaction of RanBPM with FMRP was confirmed by in vivo and in vitro assays. In brain tissue sections, RanBPM is highly expressed in the neurons of cerebral cortex and the cerebellar purkinje cells, in a pattern similar to that described for FMRP. Sequence analysis shows that RanBPM is a multi-domain protein. The interaction with FMRP was mapped in a newly identified CRA motif present in the RanBPM C terminus. Our results suggest that the functional role of RanBPM binding is modulation of the RNA-binding properties of FMRP.
"It was previously suggested that RanBPM interacted with microtubules, but this observation was later dismissed as the original study used an antibody that did not recognize RanBPM (Nakamura et al., 1998; Nishitani et al., 2001). Some studies have subsequently suggested a potential role for RanBPM in microtubule regulation (Menon et al., 2004; Togashi et al., 2006), although a direct association of RanBPM with microtubules remains to be confirmed. Interestingly, the highly similar protein RanBP10, whose expression is restricted to hematopoietic cell lineages, has been shown to function in platelet microtubule organization through an interaction with b1-tubulin (Kunert et al., 2009; Schulze et al., 2008). "
[Show abstract][Hide abstract] ABSTRACT: In conditions of proteasomal impairment, the build-up of damaged or misfolded proteins activates a cellular response leading to the recruitment of damaged proteins into perinuclear aggregates called aggresomes. Aggresome formation involves the retrograde transport of cargo proteins along the microtubule network and is dependent on the histone deacetylase HDAC6. Here we show that ionizing radiation (IR) promotes Ran-Binding Protein M (RanBPM) relocalization into discrete perinuclear foci where it co-localizes with aggresome components ubiquitin, dynein and HDAC6, suggesting that the RanBPM perinuclear clusters correspond to aggresomes. RanBPM was also recruited to aggresomes following treatment with the proteasome inhibitor MG132 and the DNA-damaging agent etoposide. Strikingly, aggresome formation by HDAC6 was markedly impaired in RanBPM shRNA cells, but was restored by re-expression of RanBPM. RanBPM was found to interact with HDAC6 and to inhibit its deacetylase activity. This interaction was abrogated by a RanBPM deletion of its LisH/CTLH domain, which also prevented aggresome formation, suggesting that RanBPM promotes aggresome formation through an association with HDAC6. Our results suggest that RanBPM regulates HDAC6 activity and is a central regulator of aggresome formation.
Biology Open 05/2014; 3(6). DOI:10.1242/bio.20147021 · 2.42 Impact Factor
"The Fragile X Mental Retardation Protein (FMRP) binds APP mRNA and represses its translation (Westmark and Malter, 2007). Interestingly RanBP9 may interact with FMRP and inhibit its RNA binding activity (Menon et al., 2004). It would therefore be tempting to speculate that APP protein levels may be influenced by miR-101 post-transcriptional regulation of FMRP/RanBP9 complex. "
[Show abstract][Hide abstract] ABSTRACT: Neurodegeneration associated with amyloid β (Aβ) peptide accumulation, synaptic loss, and memory impairment are pathophysiological features of Alzheimer's disease (AD). Numerous microRNAs regulate amyloid precursor protein (APP) expression and metabolism. We previously reported that miR-101 is a negative regulator of APP expression in cultured hippocampal neurons. In this study, a search for predicted APP metabolism-associated miR-101 targets led to the identification of a conserved miR-101 binding site within the 3' untranslated region (UTR) of the mRNA encoding Ran-binding protein 9 (RanBP9). RanBP9 increases APP processing by β-amyloid converting enzyme 1 (BACE1), secretion of soluble APPβ (sAPPβ), and generation of Aβ. MiR-101 significantly reduced reporter gene expression when co-transfected with a RanBP9 3'-UTR reporter construct, while site-directed mutagenesis of the predicted miR-101 target site eliminated the reporter response. To investigate the effect of stable inhibition of miR-101 both in vitro and in vivo, a microRNA sponge was developed to bind miR-101 and derepress its targets. Four tandem bulged miR-101 responsive elements (REs), located downstream of the enhanced green fluorescence protein (EGFP) open reading frame and driven by the synapsin promoter, were placed in a lentiviral vector to create the pLSyn-miR-101 sponge. Delivery of the sponge to primary hippocampal neurons significantly increased both APP and RanBP9 expression, as well as sAPPβ levels in the conditioned medium. Importantly, silencing of endogenous RanBP9 reduced sAPPβ levels in miR-101 sponge-containing hippocampal cultures, indicating that miR-101 inhibition may increase amyloidogenic processing of APP by RanBP9. Lastly, the impact of miR-101 on its targets was demonstrated in vivo by intrahippocampal injection of the pLSyn-miR-101 sponge into C57BL6 mice. This study thus provides the basis for studying the consequences of long-term miR-101 inhibition on the pathology of AD.
"The CRA domain was identified as a 100 aa region at the C-terminus of RanBP9 that contains 6 α-helical regions and is conserved between RanBP9 and TWA1 . It is now appreciated that the CRA domain is also present in MAEA and Rmnd5 (Fig. 1) . "
[Show abstract][Hide abstract] ABSTRACT: Ubiquitination is an essential post-translational modification that regulates signalling and protein turnover in eukaryotic cells. Specificity of ubiquitination is driven by ubiquitin E3 ligases, many of which remain poorly understood. One such is the mammalian muskelin/RanBP9/CTLH complex that includes eight proteins, five of which (RanBP9/RanBPM, TWA1, MAEA, Rmnd5 and muskelin), share striking similarities of domain architecture and have been implicated in regulation of cell organisation. In budding yeast, the homologous GID complex acts to down-regulate gluconeogenesis. In both complexes, Rmnd5/GID2 corresponds to a RING ubiquitin ligase. To better understand this E3 ligase system, we conducted molecular phylogenetic and sequence analyses of the related components. TWA1, Rmnd5, MAEA and WDR26 are conserved throughout all eukaryotic supergroups, albeit WDR26 was not identified in Rhizaria. RanBPM is absent from Excavates and from some sub-lineages. Armc8 and c17orf39 were represented across unikonts but in bikonts were identified only in Viridiplantae and in O. trifallax within alveolates. Muskelin is present only in Opisthokonts. Phylogenetic and sequence analyses of the shared LisH and CTLH domains of RanBPM, TWA1, MAEA and Rmnd5 revealed closer relationships and profiles of conserved residues between, respectively, Rmnd5 and MAEA, and RanBPM and TWA1. Rmnd5 and MAEA are also related by the presence of conserved, variant RING domains. Examination of how N- or C-terminal domain deletions alter the sub-cellular localisation of each protein in mammalian cells identified distinct contributions of the LisH domains to protein localisation or folding/stability. In conclusion, all components except muskelin are inferred to have been present in the last eukaryotic common ancestor. Diversification of this ligase complex in different eukaryotic lineages may result from the apparently fast evolution of RanBPM, differing requirements for WDR26, Armc8 or c17orf39, and the origin of muskelin in opisthokonts as a RanBPM-binding protein.
PLoS ONE 11/2013; 8(10):e75217. DOI:10.1371/journal.pone.0075217 · 3.23 Impact Factor
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