Rules of engagement promote polarity in RNA trafficking

Many cell biological pathways exhibit overall polarity (net movement of molecules in one direction) even though individual molecular interactions in the pathway are freely reversible. The A2 RNA trafficking pathway exhibits polarity in moving specific RNA molecules from the nucleus to localization sites in the myelin compartment of oligodendrocytes or dendritic spines in neurons. The A2 pathway is mediated by a ubiquitously expressed trans-acting trafficking factor (hnRNP A2) that interacts with a specific 11 nucleotide cis-acting trafficking sequence termed the A2 response element (A2RE) found in several localized RNAs. Five different molecular partners for hnRNP A2 have been identified in the A2 pathway: hnRNP A2 itself, transportin, A2RE RNA, TOG (tumor overexpressed gene) and hnRNP E1, each playing a key role in one particular step of the A2 pathway. Sequential interactions of hnRNP A2 with different molecular partners at each step mediate directed movement of trafficking intermediates along the pathway. Specific "rules of engagement" (both and, either or, only if) govern sequential interactions of hnRNP A2 with each of its molecular partners. Rules of engagement are defined experimentally using three component binding assays to measure differential binding of hnRNP A2 to one partner in the presence of each of the other partners in the pathway. Here we describe rules of engagement for hnRNP A2 binding to each of its molecular partners and discuss how these rules of engagement promote polarity in the A2 RNA trafficking pathway. For molecules with multiple binding partners, specific rules of engagement govern different molecular interactions. Rules of engagement are ultimately determined by structural relationships between binding sites on individual molecules. In the A2 RNA trafficking pathway rules of engagement governing interactions of hnRNP A2 with different binding partners provide the basis for polarity of movement of intermediates along the pathway

Formation of Trans-Activation Competent HIV-1 Rev:RRE Complexes Requires the Recruitment of Multiple Protein Activation Domains

The HIV-1 Rev trans-activator is a nucleocytoplasmic shuttle protein that is essential for virus replication. Rev directly binds to unspliced and incompletely spliced viral RNA via the cis-acting Rev Response Element (RRE) sequence. Subsequently, Rev oligomerizes cooperatively and interacts with the cellular nuclear export receptor CRM1. In addition to mediating nuclear RNA export, Rev also affects the stability, translation and packaging of Rev-bound viral transcripts. Although it is established that Rev function requires the multimeric assembly of Rev molecules on the RRE, relatively little is known about how many Rev monomers are sufficient to form a trans-activation competent Rev:RRE complex, or which specific activity of Rev is affected by its oligomerization. We here analyzed by functional studies how homooligomer formation of Rev affects the trans-activation capacity of this essential HIV-1 regulatory protein. In a gain-of-function approach, we fused various heterologous dimerization domains to an otherwise oligomerization-defective Rev mutant and were able to demonstrate that oligomerization of Rev is not required per se for the nuclear export of this viral trans-activator. In contrast, however, the formation of Rev oligomers on the RRE is a precondition to trans-activation by directly affecting the nuclear export of Rev-regulated mRNA. Moreover, experimental evidence is provided showing that at least two protein activation domains are required for the formation of trans-activation competent Rev:RRE complexes. The presented data further refine the model of Rev trans-activation by directly demonstrating that Rev oligomerization on the RRE, thereby recruiting at least two protein activation domains, is required for nuclear export of unspliced and incompletely spliced viral RNA

Control of Flowering and Cell Fate by LIF2, an RNA Binding Partner of the Polycomb Complex Component LHP1

Polycomb Repressive Complexes (PRC) modulate the epigenetic status of key cell fate and developmental regulators in eukaryotes. The chromo domain protein LIKE HETEROCHROMATIN PROTEIN1 (LHP1) is a subunit of a plant PRC1-like complex in Arabidopsis thaliana and recognizes histone H3 lysine 27 trimethylation, a silencing epigenetic mark deposited by the PRC2 complex. We have identified and studied an LHP1-Interacting Factor2 (LIF2). LIF2 protein has RNA recognition motifs and belongs to the large hnRNP protein family, which is involved in RNA processing. LIF2 interacts in vivo, in the cell nucleus, with the LHP1 chromo shadow domain. Expression of LIF2 was detected predominantly in vascular and meristematic tissues. Loss-of-function of LIF2 modifies flowering time, floral developmental homeostasis and gynoecium growth determination. lif2 ovaries have indeterminate growth and produce ectopic inflorescences with severely affected flowers showing proliferation of ectopic stigmatic papillae and ovules in short-day conditions. To look at how LIF2 acts relative to LHP1, we conducted transcriptome analyses in lif2 and lhp1 and identified a common set of deregulated genes, which showed significant enrichment in stress-response genes. By comparing expression of LHP1 targets in lif2, lhp1 and lif2 lhp1 mutants we showed that LIF2 can either antagonize or act with LHP1. Interestingly, repression of the FLC floral transcriptional regulator in lif2 mutant is accompanied by an increase in H3K27 trimethylation at the locus, without any change in LHP1 binding, suggesting that LHP1 is targeted independently from LIF2 and that LHP1 binding does not strictly correlate with gene expression. LIF2, involved in cell identity and cell fate decision, may modulate the activity of LHP1 at specific loci, during specific developmental windows or in response to environmental cues that control cell fate determination. These results highlight a novel link between plant RNA processing and Polycomb regulation

The rheumatoid arthritis-associated autoantigen hnRNP-A2 (RA33) is a major stimulator of autoimmunity in rats with pristane-induced arthritis

A single intradermal injection of the mineral oil pristane in susceptible DA.1F rats induces erosive arthritis closely mimicking rheumatoid arthritis (RA). Pristane-induced arthritis (PIA) is driven by autoreactive T cells but no autoantigen has been identified to date. We therefore analyzed B and T cell responses to autoantigens potentially involved in the pathogenesis of RA, including IgG, citrullinated proteins, stress proteins, glucose-6-phosphate isomerase, and heterogeneous nuclear ribonucleoprotein (hnRNP)-A2 (RA33). IgG and lgM autoantibodies to hnRNP-A2 were detectable in sera of pristane-primed DA.1F rats already 1 wk before disease onset, reached maximum levels during the acute phase, and correlated with arthritis severity. Apart from rheumatoid factor, autoantibodies to other Ags were not observed. CD4(+) lymph node cells isolated 10 days after pristane injection produced IFN-gamma but not IL-4 in response to stimulation with hnRNP-A2, whereas none of the other candidate Ags elicited cytokine secretion. Surprisingly, hnRNP-A2 also stimulated lymph node cells of naive animals to produce inflammatory cytokines in a MyD88-dependent manner. Furthermore, hnRNP-A2 was highly overexpressed in the joints of rats injected with pristane. Overexpression coincided with the appearance of anti-RA33 Abs and preceded the onset of clinical symptoms of PIA by several days. Taken together, these data suggest hnRNP-A2 to be among the primary inducers of autoimmunity in PIA. Therefore, this Ag might play a pivotal role in the pathogenesis of PIA and possibly also human RA