The membrane-bound proteins of periplasmic permeases form a complex. Identification of the histidine permease HisQMP...

The membrane-bound proteins of periplasmic transport systems have been hypothesized to form a complex with relatively little experimental support. Here we present experimental evidence that HisQ, HisM, and HisP, the membrane-bound proteins of the periplasmic histidine transport system of Salmonella typhimurium, form such a complex. We have developed antibodies specific to each of these proteins to aid in their characterization. Extractions with urea, alkaline pH, or Triton X-114 show that HisQ and HisM are integral membrane proteins. By these tests HisP displays an unusual behavior, being associated with the membrane whether or not HisQ and HisM are present and despite its hydrophilic sequence. However, the nature of HisPs interaction with the membrane is shown to vary depending on the presence of HisQ and HisM. In their absence, HisP is somewhat peripherally associated with the membrane, while in their presence it binds much more tightly, indicating that it forms a complex in association with HisQ and HisM. This is demonstrated by the coimmunoprecipitation of all three proteins by antibodies directed against any one of them. Chemical cross-linking allowed the characterization of the subunit stoichiometry of the complex as two HisPs to one HisQ and one HisM. Within this complex all three proteins probably contact each other and the two HisPs form a dimer. We hypothesize that HisQ and HisM with their multiple membrane-spanning segments form a "channel" within which the HisP subunits are located.