Target specificity of neuronal RNA-binding protein, Mel-N1: direct binding to the 3' untranslated region of its own mRNA.

We have identified cDNAs encoding Mel-N1, the mouse homologue of a human nervous system-specific RNA-binding protein, Hel-N1. Two major mRNA transcripts of Mel-N1 were detected predominantly in the adult mouse brain by Northern blot analysis. To gain insight into the RNA binding specificity of Mel-N1, we performed iterative in vitro RNA selection. The resulting in vitro selected RNAs were found to contain AU-rich sequences as well as a GAAA motif in the majority of clones. By means of in vitro binding assays we demonstrate that this GAAA sequence appears to significantly affect the Mel-N1 RNA-binding efficiency. Our studies further reveal that Mel-N1 can bind to its own 3' untranslated region (3'UTR) as well as to the c-fos 3'UTR, and is localized predominantly in the cytoplasmic region in cells, suggesting that posttranscriptional autoregulation of Mel-N1 gene expression occurs in vivo.