3.A.1.3.10
Cystine/cysteine/diaminopimelate transporter, CysXYZ; these proteins are also designated FliY/YecS/YecC.  Note, another transporter is designated CysZ in E. coli (TC# 2.A.121.1.1).  CysXYZ also transports the toxic amino acid analogues, L-selenaproline (SCA; L-selenazolidine-4-carboxylic acid) and L-selenocystine (SeCys) (Deutch et al. 2014). FliY binds L-cystine, L-cysteine, and D-cysteine with micromolar affinities, but binding of the L- and D-enantiomers induced different conformational changes in FliY, where the L- enantiomer/SBP complex interacted more efficiently with the YecSC transporter. YecSC has low basal ATPase activity that is moderately stimulated by apo-FliY, more strongly by D-cysteine-bound FliY, and maximally by L-cysteine- or L-cystine-bound FliY (Sabrialabe et al. 2020). FliY may exist in a conformational equilibrium between an open, unliganded form that does not bind to the YecSC transporter and closed, unliganded and closed, liganded forms that bind this transporter with variable affinities but equally stimulate its ATPase activity. These findings differ from previous observations for similar ABC transporters, highlighting the extent of mechanistic diversity in this large protein family (Sabrialabe et al. 2020).

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