Figure 4.

TTX binding to cATPase. The upper mass spectra were acquired before (left) and after (right) incubation with TTX. Without TTX, the dominant complex species is the F₁–δ‐cATPase. The intact F₁ and F₁–δ–ε complexes are present at lower intensities. The intensity of the intact F₁‐cATPase increased after TTX binding, leading to two main complexes, the F₁‐cATPase and the F₁–δ‐cATPase. Upon CID fragmentation (lower mass spectra), the ε‐subunit dissociates, mainly yielding the F₁–δ–ε‐cATPase without TTX (left). CID fragmentation of the TTX‐bound cATPase results in an intensity change of the CID products. Loss of the ε‐subunit in the TTX‐bound complex leads primarily to formation of the F₁–ε complex. The crystal structures of the spinach ATPase with (Protein Data Bank ID PDB 1KMH, right) and without (Protein Data Bank ID PDB 1FX0, left) TTX, as well as the structure of TTX, are shown.