Register and chain-tracing errors in the β-strands of the barrel are actually not a problem, because the first and last stretches of each strand are fixed in the x-ray structure of domain 3. Together with the hydrophobic residues facing the lipid bilayer on the outside of the barrel and the polar and charged residues facing the inside, this leaves virtually no room for chain trace errors even at 4.5 Å. Moreover, we know that the β-strands in the barrel are well-resolved because of the sharp peak at 4.8 Å in the FSC curve (Figure 1—figure supplement 2B), which is the characteristic distance between β-strands. We can therefore be sure that our atomic model of the β-barrel, which is the major new part of the structure, is correct and accurate within the constraints of the map.