Heterogeneous Nuclear Ribonucleoprotein (hnRNP) E1 Binds to hnRNP A2 and Inhibits Translation of A2 Response Element mRNAs

Abstract

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.

Multiplexed Dendritic Targeting of α Calcium Calmodulin-dependent Protein Kinase II, Neurogranin, and Activity-regulated Cytoskeleton-associated Protein RNAs by the A2 Pathway

“…This marks the RNA for subsequent targeting by the A2 pathway when it reaches the cytoplasm. Binding of hnRNP A2 to exogenous RNAs microinjected into the cytoplasm occurs because the concentration of hnRNP A2 in the cytoplasm still exceeds the K D for binding to A2RE sequences (Kosturko et al, 2006). Linkage combines multiple A2RE RNAs into composite granules.…”

“…Suppression prevents translation of RNAs in granules during transmission from nucleus to synapse. This may be accomplished by hnRNP E1, which is a granule component that binds to hnRNP A2 and inhibits translation of A2RE RNAs (Kosturko et al, 2006).…”

“…RNAs whose dendritic targeting is inhibited by excess A2RE RNA are likely to be targeted to dendrites by the A2 pathway. The rate-limiting component in the A2 pathway may not be hnRNP A2 itself, which is present in very high concentrations in most cells (Kosturko et al, 2006). A more likely possibility is TOG protein, which is present in lower concentrations in the cytoplasm where it binds to hnRNP A2 and is a component of RNA granules (Kosturko et al, 2005).…”

“…Ligand consisted of full-length recombinant hnRNP A2, prepared as described previously (Kosturko et al, 2006) immobilized by covalent carbodiimide coupling to a CM5 chip (Biacore, Uppsala, Sweden). The amount of protein immobilized on the chip was 5000 resonance units (RU), corresponding to 5 ng/mm 2 .…”

Heterogeneous Nuclear Ribonucleoprotein (hnRNP) F Is a Novel Component of Oligodendroglial RNA Transport Granules Contributing to Regulation of Myelin Basic Protein (MBP) Synthesis

“…During granule transport, MBP mRNA is maintained in a translationally silenced state, which has been proposed to be regulated by hnRNP E1 that is recruited to RNA granules by hnRNP A2 (Fig. 8) (32). Src family kinases have been shown to regulate translation initiation of mRNAs transported in RNA granules in both neurons and erythrocytes by phosphorylating distinct RNA-binding proteins (34,39).…”