Bruno Protein Contains an Expanded RNA Recognition Motif

The RNA recognition motif (or RRM) is a ubiquitous RNA-binding module present in ∼2% of the proteins encoded in the human genome. This work characterizes an expanded RRM, which is present in the Drosophila Bruno protein, and targets regulatory elements in the oskar mRNA through which Bruno controls translation. In this Bruno RRM, the deletion of 40 amino acids prior to the N-terminus of the canonical RRM resulted in a significantly decreased affinity of the protein for its RNA target. NMR spectroscopy showed that the expanded Bruno RRM contains the familiar RRM fold of four antiparallel β-strands and two α-helices, preceded by a 10-residue loop that contacts helix α1 and strand β2; additional amino acids at the N-terminus of the domain are relatively flexible in solution. NMR results also showed that a truncated form of the Bruno RRM, lacking the flexible N-terminal amino acids, forms a stable and complete canonical RRM, so that the loss of RNA binding activity cannot be attributed to disruption of the RR...