Identification and characterization of a novel organelle gathered around centrosomes in a microtubule-dependent manner

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mRNA transport and subsequent local translation of the mRNAs in neuronal dendrites are widely believed to be essential for local regulation of synaptic efficacy. The dendritic mRNAs are transported in densely packed granules containing the mRNAs and ribosomes, called RNA granules.RNG105 (RNA granule protein 105) is a novel protein we originally identified as a centrosomes- and microtubule-associated protein. Expression of RNG105 protein in cultured cells induces formation of cytoplasmic granules containing ribosomes and mRNAs. RNG105 was expressed highly in neuronal dendrites. These characteristics of RNG105 protein suggested that the RNG105-induced granules may equivalent to RNA granules in neurons. Immunostaining of rat hippocampal slices showed that RNG105 was associated with granules containing ribosomes and mRNAs in neuronal dendrites. Staufen, a protein for mRNA transport, and FMRP, a protein for translational regulation, were identified as the components of the granule. Microtubule- and actin-dependent motors, kinesin and myosin, were also the components of the granule. Furthermore, mRNAs associated with the granule were identified. Many of the mRNAs, such as CaM kinase IIα, CREB and BDNF mRNAs, were the key molecules for long-term synaptic plasticity. These lines of evidence indicated that RNG105 is a novel protein associated with mRNA-transporting granules which may be responsible for synaptic plasticity in neuronal dendrites.