Distribution of slow motions. (**A**) Plot of *R*_{ex} as a function of residue number of the disulfide-bond-tethered complex of Tom20 and the presequence. The yellow bar represents the *R*_{ex} values estimated from the model-free analysis, and the green line with the closed circles is *R*_{ex} from the measurement of transverse CSA/dipolar cross-relaxation rates (η_{xy}). The broken line indicates the threshold for the *R*_{ex} mapping on the three-dimensional structures. The positions of proline residues are indicated with the letter ‘p'. (**B**) Open-book representations of the spatial distributions of amide ^{15}N spins with an *R*_{ex} value larger than 1 s^{−1} from the η_{xy} measurement in the two crystal structures. The amide nitrogen atoms, shown as blue spheres, of Ser16′, Leu19′, and Ser20′ of the presequence peptide and of Ile74 and Gln75 of Tom20 are located at the hydrophobic contact surface (red patches on the ribbons). The amide nitrogen atoms, shown as cyan spheres, of Gly23′, Ile97, and Leu126 probably correspond to the conformational changes accompanying the motion of the presequence peptide. Finally, the yellow sphere represents the amide nitrogen atom of Gly101, which shows exceptional discrepancy between the two *R*_{ex} values. This could arise from the variation in the *R*_{ex} value calculated from η_{xy} due to the variation in magnitude and orientation of the CSA tensor. Note that the large values of the three ^{15}N spins at positions 16, 19, and 20 of the presequence suggest a very interesting rule: an amide ^{15}N spin in an α-helical conformation monitors the side-chain motion of the preceding residue, in the present case, Leu15′, Leu18′, and Leu19′. (**C**) Plot of *R*_{ex} as a function of residue number of the free form of Tom20. The yellow bar represents the *R*_{ex} values estimated from the model-free analysis, and the green line with the closed circles represents *R*_{ex} from the η_{xy} measurement. The broken line indicates the threshold for the *R*_{ex} mapping on the three-dimensional structure. The positions of proline residues are indicated with the letter ‘p'. (**D**) The spatial distribution of amide ^{15}N spins with an *R*_{ex} value larger than 1 s^{−1} from the η_{xy} measurement. The Tom20 structure in the A-linker crystal structure was used instead of the free structure. The amide nitrogen atoms with large *R*_{ex} values, shown as green spheres, are widely distributed in the Tom20 structure.

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