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Polyadenylation is a nuclear process that involves the endonucleolytic cleavage of RNA transcripts and the addition of poly(A) tails. The cleavage often takes place at different positions within the same RNA transcript, generating alternative 3’ends. Polyadenylation regulates mRNA localization, stability and translation and is likely to regulate complex processes such as synapse formation, synaptic plasticity and memory. Here we examined whether PolyA Binding Protein Nuclear 1 (Pabpn1), an RNA binding protein known to regulate alternative polyadenylation and polyA tail length in other systems, regulates neuronal mRNA function. Using immunocytochemistry we determined that Pabpn1 is present in both hippocampal slices and cultured hippocampal neurons. Applying shRNAs to knock-down Pabpn1 we discovered that Pabpn1 regulates the mRNA abundance and localization of key synaptic plasticity genes including Calcium/Calmodulin Dependent Protein Kinase II Alpha (Camk2a) and Glutamate Ionotropic Receptor AMPA Type Subunit 2 (Gria2). Furthermore, Pabpn1 knock-down prevented the homeostatic scaling of synaptic transmission elicited by bicuculline. These data demonstrate a link between Pabpn1, polyadenylation and neuronal plasticity.

The 5′ cap and 3′ poly(A) tail of eukaryotic mRNAs act synergistically to enhance translation. This synergy is mediated via interactions between eIF4G (a component of the eIF4F cap binding complex) and poly(A) binding protein (PABP). Paip2 (PABP-interacting protein 2) binds PABP and inhibits translation both in vitro and in vivo by decreasing the affinity of PABP for polyadenylated RNA. Here, we describe the functional characteristics of Paip2B, a Paip2 homolog. A full-length brain cDNA of Paip2B encodes a protein that shares 59% identity and 80% similarity with Paip2 (Paip2A), with the highest conservation in the two PABP binding domains. Paip2B acts in a manner similar to Paip2A to inhibit translation of capped and polyadenylated mRNAs both in vitro and in vivo by displacing PABP from the poly(A) tail. Also, similar to Paip2A, Paip2B does not affect the translation mediated by the internal ribosome entry site (IRES) of hepatitis C virus (HCV). However, Paip2A and Paip2B differ with respect to both mRNA and protein distribution in different tissues and cell lines. Paip2A is more highly ubiquitinated than is Paip2B and is degraded more rapidly by the proteasome. Paip2 protein degradation may constitute a primary mechanism by which cells regulate PABP activity in translation.

Poly(A)-binding protein II (PABP2) is an abundant nuclear protein that binds with high affinity to nascent poly(A) tails, stimulating their extension and controlling their length. In the cytoplasm, a distinct protein (PABP1) binds to poly(A) tails and participates in mRNA translation and stability. How cytoplasmic PABP1 substitutes for nuclear PABP2 is still unknown. Here we report that PABP2 shuttles back and forth between nucleus and cytoplasm by a carrier-mediated mechanism. A potential novel type of nuclear localization signal exists at the C-terminus of the protein, a domain that is highly enriched in methylated arginines. PABP2 binds directly to transportin in a RanGTP-sensitive manner, suggesting an involvement of this transport receptor in mediating import of the protein into the nucleus. Although PABP2 is small enough to diffuse passively through the nuclear pores, protein fusion experiments reveal the existence of a facilitated export pathway. Accordingly, no transport of PABP2 to the cytoplasm occurs at 4 degrees C. In contrast, export of PABP2 continues in the absence of transcription, indicating that transport to the cytoplasm is independent of mRNA traffic. Thus, rather than leaving the nucleus as a passive passenger of mRNAs, the data suggest that PABP2 interacts with the nuclear export machinery and may therefore contribute to mRNA transport.