a, Dipeptidase mutant (1POK E239Y) visualized by TEM with negative staining. The protein buffer was Tris 20 mM, 100 mM NaCl, pH 7.5. The protein mutant self-assembles into filaments that tend to bundle together. b, Example of electron microscopy images on the basis of which the distance separating adjacent homomers in fibres was measured. Mutants in the images are 1D7A (K11L/E22L/E25L/D158L), 2WCV (E77Y), 1L6W (K97Y/K100Y/E102Y), 1M3U (D157L/E158L/D161L), 3N75 (D460L), 2CG4 (K126Y/D131Y), and 1POK (E239Y). All the samples were in Tris 20 mM, pH 7.5 except 1POK (E239Y) which had in addition 100 mM NaCl. c, Fusion of YFP to the dipeptidase mutant (1POK E239Y) does not affect its structure when compared with the wild-type fusion, as seen by circular dichroism. The protein buffer was Tris 20 mM, pH 7.5. d, We examined the dipeptidase fibre-forming mutant fused to YFP by TEM with negative straining. The protein buffer was Tris 20 mM, 100 mM NaCl, pH 7.5. The mutant forms filaments similar to those observed without the YFP fusion.