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

June 23rd, 2014 by Yamashita, M., Shepherd, M., Booth, W. I., Xie, H., Postis, V., Nyathi, Y., Tzokov, S., Poole, R. K., Baldwin, S. A., Bullough, P. A.

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.
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