The citrate carrier CitS probed by single-molecule fluorescence spectroscopy

A prominent region of the Na(+)-dependent citrate carrier (CitS) from Klebsiella pneumoniae is the highly conserved loop X-XI, which contains a putative citrate binding site. To monitor potential conformational changes within this region by single-molecule fluorescence spectroscopy, the target cysteines C398 and C414 of the single-Cys mutants (CitS-sC398, CitS-sC414) were selectively labeled with the thiol-reactive fluorophores AlexaFluor 546/568 C(5) maleimide (AF(546), AF(568)). While both single-cysteine mutants were catalytically active citrate carriers, labeling with the fluorophore was only tolerated at C398. Upon citrate addition to the functional protein fluorophore conjugate CitS-sC398-AF(546), complete fluorescence quenching of the majority of molecules was observed, indicating a citrate-induced conformational change of the fluorophore-containing domain of CitS. This quenching was specific for the physiological substrate citrate and therefore most likely reflecting a conformational change in the citrate transport mechanism. Single-molecule studies with dual-labeled CitS-sC398-AF(546/568) and dual-color detection provided strong evidence for a homodimeric association of CitS.