Fig. S4.

Assignment of BtubA and BtubB to their corresponding map regions. Because BtubA and BtubB are similar in structure (7), we needed to use BtubC, which only binds to each BtubAB heterodimer once to keep BtubA and BtubB separate during the cryoEM helical reconstruction. We then needed to determine which of the monomers in the map are BtubA and BtubB. The subunit assignment was done based on the excellent quality of the 3.6 Å resolution map, which enabled the unambiguous assignment through inspection of many map regions. A and B show two such map regions, superimposed with both the BtubA and BtubB model from the crystal structure, fitted into the map with CHIMERA (28). BtubA map and model are in red, blue for BtubB.