Neuronal RNA granules: a link between RNA localization and stimulation-dependent translation.

RNA granules are a macromolecular structure observed in neurons, where they serve as motile units that translocate mRNAs. Isolated RNA granules are highly enriched in Staufen protein and ultrastructurally contain densely packed clusters of ribosomes. With depolarization, many mRNAs, including those involved in plasticity, rapidly shift from the RNA granule fraction to polysomes. Depolarization reorganizes granules and induces a less compact organization of their ribosomes. RNA granules are not translationally competent, as indicated by the failure to incorporate radioactive amino acids and the absence of eIF4E, 4G, and tRNAs. We concluded that RNA granules are a local storage compartment for mRNAs under translational arrest but are poised for release to actively translated pools. Local release of mRNAs and ribosomes from granules may serve as a macromolecular mechanism linking RNA localization to translation and synaptic plasticity.

Ferritin expression in maturing normal human erythroid precursors.

We studied the expression of H- and L-ferritin subunits at sequential stages of maturation of normal human erythroid precursors. The erythroid cells developed in liquid culture and were purified immunomagnetically before analysis. It was found that the content of both ferritin subunits decreased exponentially with maturation: the decrease was rapid when cellular haemoglobin was low, and it slowed down when the haemoglobin was increased. This mode of decline was especially pronounced for the L-subunits. The H-/L-subunit ratio did not change significantly during the investigated period. The synthesis of both subunits was equal at each given developmental stage, and declined significantly with maturation. However, this decline was just slightly faster than that of total protein synthesis. The data indicated that the degradation of H- and L-ferritin also declined as maturation proceeded. No decrease was observed in mRNA levels of either ferritin subunit. Thus, the ferritin content and turnover were maximal at the beginning of haemoglobin accumulation and diminished later. As the rate of ferritin turnover determines the rate of incorporation and release of its iron, the results presented suggest that ferritin mediates cellular iron transport and donates iron for haem synthesis, mainly at the beginning of haemoglobin accumulation. The synthesis of both ferritin subunits is regulated during erythroid maturation at the post-transcriptional level.

Translational control of specific genes during differentiation of HL-60 cells.

Eukaryotic gene expression can be regulated through selective translation of specific mRNA species. Nevertheless, the limited number of known examples hampers the identification of common mechanisms that regulate translation of specific groups of genes in mammalian cells. We developed a method to identify translationally regulated genes. This method was used to examine the regulation of protein synthesis in HL-60 cells undergoing monocytic differentiation. A partial screening of cellular mRNAs identified five mRNAs whose translation was specifically inhibited and five others that were activated as was indicated by their mobilization onto polysomes. The specifically inhibited mRNAs encoded ribosomal proteins, identified as members of the 5'-terminal oligopyrimidine tract mRNA family. Most of the activated transcripts represented uncharacterized genes. The most actively mobilized transcript (termed TA-40) was an untranslated 1.3-kilobase polyadenylated RNA with unusual structural features, including two Alu-like elements. Following differentiation, a significant change in the cytoplasmic distribution of Alu-containing mRNAs was observed, namely, the enhancement of Alu-containing mRNAs in the polysomes. Our findings support the notion that protein synthesis is regulated during differentiation of HL-60 cells by both global and gene-specific mechanisms and that Alu-like sequences within cytoplasmic mRNAs are involved in such specific regulation.