This Article contains errors in Fig. 1, Table 1 and the Methods section. In panel c, the labels for PmScsC and EcDsbC in the upper two curves are interchanged. In Table 1 and the Methods section entitled 'Extended structure', the space group of the extended PmScsC structure is incorrectly referred to as H32 and should read H32. Correct versions of Fig. 1 and Table 1 are presented below; the errors have not been corrected in the Article.