Extended Data Fig. 8: Acidic loops compose the center of the radial spoke head surface.

a, Loops at the head surface shown in the top right (some were removed for clarity) were modeled into a clear density observed in the cryo-EM map (‘map 1’), indicating the loops are structured. Asterisks indicate loops shown in b. b, Beta hairpins in RSP4 and RSP6, located at the head surface, are stabilized by hydrogen bonds (dashed lines). Residues that participate in these hydrogen bonds are indicated. c, Zoom into box in a, showing stabilizing lateral interactions (hydrogen bonds in dashed lines) between four different loops. The loops at the RS head surface are also supported by the tightly-packed folds that compose the RS head scaffold (Fig. 2). d, Electrostatic volume of the head surface generated in PyMol. Dashed red lines indicate regions not modeled, suggesting that the RS head surface is even more acidic than visualized by the electrostatic map. Number of missing residues and their percentage of D and E residues are listed on the right for each region.