Model of the factor relay between Nsa2, Rsa4, and the Rea1 MIDAS and its proposed function in rearranging helix 89 toward the PTC. (A) The depicted Rea1 MIDAS domain (green) carrying a coordinated cation (red ball) is a structural model (modeled with Phyre2, template PDB accession no. 4FX5). The ctRsa4 crystal structure is shown in blue, with conserved E117 orientated toward the MIDAS domain/ion. The Nsa2 N and C domains solved by NMR are shown in red (see also Fig. 3), and the Nsa2 peptide (85–95 aa) in proximity to the top site of the Rsa4 β-propeller with hydrophobic residues is shown in yellow. (B) Illustration of the proposed force relay that transmits mechano-chemical energy from Rea1 through Rsa4 and Nsa2 to helix 89.