Iron(III) hydroxamate transport in Escherichia coli K-12: FhuB-mediated membrane association of the FhuC protein and negative complementation of fhuC mutants.

Iron(III) hydroxamate transport across the cytoplasmic membrane is catalyzed by the very hydrophobic FhuB protein and the membrane-associated FhuC protein, which contains typical ATP-binding domains. Interaction between the two proteins was demonstrated by immunoelectron microscopy with anti-FhuC antibodies, which showed FhuB-mediated association of FhuC with the cytoplasmic membrane. In addition, inactive FhuC derivatives carrying single amino acid replacements in the ATP-binding domains suppressed wild-type FhuC transport activity, which arose either from displacement of active FhuC from FhuB by the mutated FhuC derivatives or from the formation of mixed inactive FhuC multimers between wild-type and mutated FhuC proteins. Inactive FhuC derivatives containing internal deletions and insertions showed no phenotypic suppression, indicating conformational alterations that rendered the FhuC derivatives unable to displace wild-type FhuC. It is concluded that the physical interaction between FhuC and FhuB implies a coordinate activity of both proteins in the transport of iron(III) hydroxamates through the cytoplasmic membrane.