Similarity of protein-RNA interfaces based on motif analysis

We have developed a method for determination of the similarity of pairs of protein-RNA complexes, which we refer to as SIMA (Similarity by Identity and Motif Alignment). The key element in the SIMA method is the description of the protein-RNA interface in terms of motifs (salt bridges, aromatic stacking interactions, nonaromatic stacks, hydrophobic interactions, and hydrogen-bonded motifs), in addition to single hydrogen bonds and van der Waals contacts. Based on a pairwise scoring function combining motif alignment with identity of the protein and RNA sequences, we define a SIMA score for any pair of protein-RNA complexes. A positive score indicates similarity between the complexes. We used the SIMA method to identify 284 nonredundant binary protein-RNA complexes out of 776 such complexes in 382 nonribosomal protein-RNA structure files obtained from the RCSB database. SIMA allows rapid and quantitative comparison of protein-RNA interfaces and may be useful for interface classification with potential functional and evolutionary implications.