The ATP-binding component of a prokaryotic traffic ATPase is exposed to the periplasmic (external) surface

The membrane-bound complex of bacterial periplasmic permeases consists of two hydrophobic integral membrane proteins and two copies of a hydrophilic ATP-binding protein. The ATP-binding proteins from all periplasmic permeases display a high level of sequence similarity and are referred to as "conserved components." The conserved component from the histidine permease, HisP, has been postulated on the basis of genetic evidence to be accessible at the exterior membrane surface, in contrast to the commonly postulated association with the interior membrane surface as peripheral membrane proteins. We have used proteolysis and biotinylation of membrane vesicles to show that HisP is accessible to these reagents at the external surface and that this orientation depends on the presence of the two hydrophobic components, HisQ and HisM. Several binding-protein-independent hisP mutants are shown to produce HisP proteins that are more susceptible to proteases from the external membrane surface. Since the hydrophilic component is well conserved also in a group of eukaryotic transporters, which together with many prokaryotic systems form the superfamily of traffic ATPases, this insight about its membrane topology has general implications for understanding the molecular mechanism of action of this large superfamily, which includes the cystic fibrosis transmembrane conductance regulator and multidrug-resistance proteins.