13CH3-S Labeling with MMTS.

¹³C-MMTS was stored at a concentration of 100 mM in DMSO at −20 °C. Labeling was achieved using an approach described previously (46). Briefly, following buffer exchange of the purified VAT mutants (K111C, M652C, and M684C) into a DTT-free buffer, ¹³C-MMTS stock solution was added at 50% molar excess (over protein) and the reaction at room temperature was left overnight. Unreacted ¹³C-MMTS was removed by buffer exchange of the product into NMR buffer [25 mM potassium phosphate, 50 mM NaCl, 4.3 mM NaN₃, 1 mM EDTA (pH 7.5) in 99.9% D₂O].