Structure and Function of the Bacterial Heterodimeric ABC Transporter CydDC: Stimulation of the ATPase Activity by Thiol and Heme Compounds

In Escherichia coli, the biogenesis of both cytochrome bd-type quinol oxidases and periplasmic cytochromes requires the ABC- type cysteine/glutathione (GSH) transporter, CydDC. Recombinant CydDC was purified as a heterodimer and found to be an active ATPase both in soluble form with detergent and when reconstituted into a lipid environment. Two dimensional crystals of CydDC were analysed by electron cryomicrosopy and the protein was shown to be made up of two non-identical domains corresponding to the putative CydD and CydC subunits, with dimensions characteristic of other ABC transporters. CydDC binds heme b. Detergent-solubilized CydDC appears to adopt at least two structural states, each associated with a characteristic level of bound heme. The purified protein in detergent showed a weak basal ATPase activity (ca. 100 nmol Pi/min/mg) that was stimulated approximately three-fold by various thiol compounds, suggesting that CydDC could act as a thiol transporter. The presence of heme (either intrinsic or added in the form of hemin) led to a further enhancement of thiol-stimulated ATPase activity although a large excess of heme inhibited activity. Similar responses of the ATPase activity were observed with CydDC reconstituted into E. coli lipids. These results suggest that heme may have a regulatory role in CydDC mediated transmembrane thiol transport.