The yeast DEAD box protein Mss116p is a general RNA chaperone that functions in 
mitochondrial group I and II intron splicing, translational activation, and RNA 
end processing. Here we determined high-resolution X-ray crystal structures of 
Mss116p complexed with an RNA oligonucleotide and ATP analogs AMP-PNP, 
ADP-BeF(3)(-), or ADP-AlF(4)(-). The structures show the entire helicase core 
acting together with a functionally important C-terminal extension. In all 
structures, the helicase core is in a closed conformation with a wedge alpha 
helix bending RNA 3' of the central bound nucleotides, as in previous DEAD box 
protein structures. Notably, Mss116p's C-terminal extension also bends RNA 5' of 
the central nucleotides, resulting in RNA crimping. Despite reported functional 
differences, we observe few structural changes in ternary complexes with 
different ATP analogs. The structures constrain models of DEAD box protein 
function and reveal a strand separation mechanism in which a protein uses two 
wedges to act as a molecular crimper.