Lambda repressor N-terminal DNA-binding domain as an assay for protein transmembrane segment interactions in vivo

To understand the determinants of membrane protein interactions, we have developed an in vivo genetic assay system for detecting homodimerization of transmembrane (TM) segments from integral membrane proteins. Our approach is to generate gene fusions between potentially dimerizing TM segments and a cytoplasmic DNA-binding protein that lacks its intrinsic dimerization domain. This genetic approach allows us to screen and distinguish among known dimerizing domains and weakly dimerizing mutants, as well as non-dimerizing TM segments. We replaced the bacteriophage lambda cI repressor C-terminal dimerization domain with the human erythrocyte glycophorin A transmembrane segment (GpA TM). GpA TM forms SDS-resistant homodimers in vitro. Expression of this membrane-associated fusion in Escherichia coli conferred the same degree of immunity to lambda cI phages as the wild-type, intact lambda repressor. Single amino acid substitutions that disrupt the GpA TM dimer interface were introduced into the lambda-GpA TM fusion proteins. These mutations dramatically reduced immunity of E. coli to lambda cI, such that the efficiency of plating these phages increased by greater than 10,000-fold over that conferred by the wild-type lambda-GpA TM fusion. Introduction of the putatively non-dimerizing first TM from E. coli MalF into the lambda-TM fusion vector resulted in no immunity to lambda cI phages. Fusion of the homodimeric, periplasmically localized, mature alkaline phosphatase domain to the C terminus of the lambda-TM fusion proteins containing weakly to non-dimerizing TM segments restored immunity to lambda cI phages. Results from this in vivo genetic assay system demonstrate that (1) dimerization of the lambda cI DNA-binding domain can be promoted by dimerizing TM segments, (2) strongly, weakly, and non-dimerizing TM segments can be distinguished on the basis of their ability to confer immunity to lambda cI phages, and (3) introduction of a dimerizing periplasmic domain can provide functionality to lambda-TM fusions containing weakly to non-dimerizing TM segments.