Homes for the orphans: utilization of multiple substrate-binding proteins by ABC transporters

Acquiring nutrients from the environment is essential for all microbes, and the ATP-binding cassette (ABC) transporters are one of the major routes by which bacteria achieve it. In this issue of Molecular Microbiology, Chen et al. describe their characterization of what appeared at first glance a simple ABC transporter for acquisition of quaternary ammonium compounds (QACs) in Pseudomonas sp., but their persistence in fully determining the properties of this system led to the experimental demonstration that QAC uptake utilizes three different substrate-binding proteins (SBPs), two of which are encoded at remote locations on the genome as 'orphan' SBPs that are each able to function with a single core ABC transporter. Building on the unusual nature of this system, in which multiple SBPs with non-overlapping substrate specificities compete for the same transporter binding site, they designed elegant in vivo experiments that suggest that only substrate-bound SBPs are able to form functional complexes with the membrane domains. This new finding provides an important piece of in vivo data leading to further insight into how this ubiquitous family of transporters operates.