Figure 6. Schematic model of the structural rearrangements in N130 associated with liquid-liquid phase separation in the presence of the multivalent rpL5 peptide.
(a) Structural representation of results from NMR and smFRET that revealed dramatic changes in dynamics and spatial orientation of the A2 track of N130 upon rpL5 peptide binding: apo N130 (top) and N130 in complex with rpL5 (bottom). Residues within the A2 tract of N130 are shown as colored spheres with diameters proportional to the value for the 15N nucleus of amide group. The sites of fluorescent labeling, Q15C and S125C, are indicated as purple and yellow spheres, respectively (the spheres for Q15C do not encode dynamic information). In the apo state, A2 randomly samples relatively compact conformations, while in liquid-like droplets, these residues extend away from the N130 core. (b) Schematic representation of apo N130 (i) and the proposed structural model of phase separation. Upon saturation of the two principal binding sites within the A1 and A2 tracts on N130 (ii), bound rpL5 peptides extend toward and engage in weak interactions with neighboring N130 pentamers (iii), thus creating 3D, expandable cross-links. In (iii), only a subset of the possible inter-N130 pentamer crosslinks are shown for clarity. We suggest that one of the correlation distances observed using SANS (77 Å) corresponds to the inter-N130 pentamer spacing.