Identification of in vivo RNA tragets of the RNA-binding proteins Acinus and hnRNP A1
Long, Jennifer Connie
RNA-binding proteins play a central role in the post-transcriptional regulation of gene expression; however, little is known about the endogenous transcripts to which they bind. Here, I have used the ultra-violet cross-linking and immuno-precipitation (CLIP) technique to identify RNA targets directly bound to two RNA-binding proteins: Acinus and hnRNP A1. Acinus (apoptotic chromatin condensation inducer in the nucleus) contains a region that is homologous to the RNA binding domain of the Drosophila splicing regulator sex-lethal, and a serine and arginine rich region similar to that seen in the SR family of proteins, which function extensively in splicing. Furthermore it is a component of the multi-protein spliceosome complex, and I have demonstrated it can directly bind polyadenylated RNA. I have shown that Acinus displays a diffuse nuclear localisation pattern, however, overexpression of an epitope-tagged protein results in its accumulation in enlarged nuclear speckles. Together these results suggest a role in pre-mRNA splicing. Acinus is cleaved during apoptosis by caspase-3, resulting in a truncated protein with chromatin condensation inducing activity (Sahara et al., 1999). Accordingly, I have demonstrated that overexpression of epitope-tagged Acinus results in an increased number of cells exhibiting an apoptotic phenotype. The proteolytic fragment contains the RNA binding region, and to determine if the role of Acinus in apoptosis is mediated by RNA interactions I utilised CLIP to identify in vivo RNA targets. I have identified several mRNA targets of Acinus and found that the binding sites in those mRNA targets predominantly map to constitutively expressed exons. This is in agreement with the exon junction complex, of which Acinus is a component, being deposited on mRNAs after splicing. These results may indicate that Acinus is a core RNA binding factor of the exon junction complex. To complement this approach, I also performed CLIP with a known alternative splicing regulator, hnRNP A1. In this manner, the binding site preferences could be compared between the two proteins. As expected, the majority of hnRNP A1 binding sites are located in introns, corresponding with their identified role of antagonizing pre-mRNA splicing by binding intronic splicing elements. Interestingly, a number of the CLIP tags are located in, or adjacent to, alternatively spliced events suggesting a role for hnRNP A1 in the regulation of alternative splicing of these specific pre-mRNAs. In addition to pre-mRNA splicing hnRNP A1 also functions in the cellular stress response. Upon environmental stresses it relocates to the cytoplasm and accumulates in cytoplasmic foci known as stress granules (Guil et al., 2006). Here I show some of the targets identified by CLIP are regulated by hnRNP A1 in times of cellular stress. In summary, I have identified two novel subsets of RNAs, bound by Acinus or hnRNP A1 in vivo. I have shown these proteins exhibit distinct binding preferences, which correspond to their biological function. This work is consistent with hnRNP A1 acting as an alternative splicing regulator, and provides evidence for a dual role of Acinus in mRNA splicing and apoptosis. This study also demonstrates the power of the CLIP technique, as identification of in vivo RNA targets allows greater understanding of the mechanisms by which RNA-binding proteins exert their regulatory control.