Functional specificity of shuttling hnRNPs revealed by genome-wide analysis of their RNA binding profiles

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Abstract
Nab2, Npl3, and Nab4/Hrp1 are essential RNA binding proteins of the shuttling hnRNP class that are required for the efficient export of mRNA. To characterize the in vivo transcript specificity of these proteins, we identified their mRNA binding partners using a microarray-based assay. Each of the three proteins was coimmunoprecipitated with many different mRNA transcripts. Interestingly, each protein exhibits preferential associations with a distinct set of mRNAs. Notably, some of these appear to denote specific functional classes. For example, the ribosomal protein mRNAs and other highly expressed transcripts significantly favor association with Npl3 over Nab2, and Nab4/Hrp1 is strongly enriched with transcripts required for amino acid metabolism. Significantly, nab4 mutants showed a striking, desensitized growth phenotype when exposed to amino acid stress conditions suggesting a biological consequence to the associations we observed. Supporting the hypothesis that these proteins display transcript specificity, we identified a unique 7-nucleotide sequence overrepresented in the transcripts highly associated with Nab2 and Nab4/Hrp1 using the REDUCE algorithm. Validating our approach, our bioinformatics analysis correctly identified the known binding site for Nab4/Hrp1. These specialized associations of the hnRNP proteins of Saccharomyces cerevisiae suggest the opportunity to regulate the processing of particular transcripts between transcription and translation.