Fig. 3: The interactions between SPM1, TrxL1/2, and tubulin.

a Cutaway view of the cryo-EM density showing a short α-helix (red dashed circle) of SPM1 (pink) bound at the intra-dimer interface between α- and β-tubulin (green and gray, respectively). b Cutaway view of the cryo-EM density of FAP363, a Mn-motif containing protein, bound to the Chlamydomonas ciliary doublet MT²⁵. c 90-degree rotation of panel a, showing SPM1 is sandwiched between tubulin and TrxL1. d Same view as panel c, but only displaying the atomic models. The two conserved residues (characteristic of the Mn motif) of SPM1 are marked by yellow stars (same in panel e). We arbitrarily assigned the protein sequence of the SPM1 density to be R4 for the purpose of model building. e Above: schematic of the domain organization of SPM1. NT: N-terminal domain, NTC: N-terminal conserved domain, CTC: C-terminal conserved domain. Below: sequence alignment of the six internal repeats (R1-R6) of SPM1. f Luminal view of the MIP densities at protofilaments 11–13 (P11–13). Unsharpened cryo-EM density map is used to better visualize the connectivity of SPM1. The red arrows point to a region of SPM1 with weaker densities (except for the SPM1 on P12), presumably due to incoherent averaging of divergent residues between R1-R6. g Schematic of the MT lumen that is cut open and unfurled at the seam.