The protein was eluted using 400 m M imidazole in the same
buffer and dialysed against buffer C (25 m M CHES, p H 9.2, 50 m M Na Cl and
2 m M EDTA) to remove imidazole. After dialysis, the sample was subjected to
ion-exchange purification using the Hi Trap Q HP (1 ml) column (GE Healthcare)
in buffer C with added 0.48 m M foscholine-14. The protein was eluted using a Na Cl
gradient (0.05–1.00 M in 20 ml). Finally, the protein was further separated by size
exclusion using the Superdex 200 10/300 GL column (GE Healthcare) in buffer
D (20 m M MES, p H 6.4, 75 m M Na Cl, 0.48 m M foscholine-14, 0.3 m M Na N
3
, and
2 m M EDTA). The elution peak fractions were analysed by SDS–PAGE (Extended
Data Fig. 2) and pooled and concentrated to achieve 0.8 m M c MCU-
Δ
NTD (mon-
omer) for NMR measurements. The final NMR sample buffer contains 20 m M
MES, p H 6.4, 75 m M Na Cl,
~
27 m M foscholine-14, 0.3 m M Na N
3
, 2 m M EDTA
and 5% D
2
O. Typical final yields of c MCU-
Δ
NTD were 2–3 mg of protein from
1 l of cell culture.