Molecular interactions in ribose transport: the binding protein module symmetrically associates with the homodimeric membrane transporter

The Escherichia coli high-affinity ribose transporter is composed of the pe riplasmic ribose-binding protein (RBP or RbsB), the membrane component (Rhs C) and the ATP-binding protein (RbsA), In order to dissect the molecular in teractions initiating the transport process, RbsC suppressors for transport -defective rbsB mutations were isolated. These suppressors are localized in two regions of RbsC, which are allele-specific to N- or C-terminal domain mutations of REP, suggesting that there are two distinct regions of RbsC, e ach interacting with one of the two domains of REP. To demonstrate that the se two regions provide a homodimeric binding surface for REP we constructed a dimeric rbsC in which two genes are joined tandemly from head to tail wi th the addition of a linker, The dimeric RbsC protein is stable and functio nal in growth and ribose uptake. By exploiting the allele specificity betwe en the domain-specific mutations and their suppressors, we generated all mu tation-suppressor combinations in a single rbsB plus the dimeric rbsC genes . Their phenotypes are consistent with the proposal that the binding protei n module interacts symmetrically with homodimeric RbsC. The mode of associa tion proposed here for the ribose transport components could be extended to other ABC transporters with similar structural organizations.