The nMS spectra were visualized using Thermo Xcalibur Qual Browser (versions 3.0.63 and 4.2.27), deconvolved using UniDec versions 3.2 and 4.1 (Marty et al., 2015 (link); Reid et al., 2019 (link)) and plotted using the m/z software (Proteometrics LLC, New York, NY). Experimental masses (Figure 1c) were reported as the average mass ± standard deviation across all the calculated mass values obtained within the observed charge state distribution.
The local resolution of the cryo-EM maps (Figure S4b) was estimated using blocres (Cardone et al., 2013 (link)) with the following parameters: box size 15, verbose 7, sampling 1.3, and cutoff 0.5. The quantification and statistical analyses for model refinement and validation were generated using MolProbity (Chen et al., 2010 (link)) and PHENIX (Adams et al., 2010 (link)).