Genetic systems for monitoring interactions of transmembrane domains in bacterial membranes

In recent years several systems have been developed to study interactions of TM domains within the inner membrane of the Gram-negative bacterium Escherichia coli. Mostly, a transmembrane domain of interest is fused to a soluble DNA-binding domain, which dimerizes in E. coli cytoplasm after interactions of the transmembrane domains. The dimeric DNA-binding domain subsequently binds to a promoter/operator region and thereby activates or represses a reporter gene. In 1996 the first bacterial system has been introduced to measure interactions of TM helices within a bacterial membrane, which is based on fusion of a transmembrane helix of interest to the DNA-binding domain of the Vibrio cholerae ToxR protein. Interaction of a transmembrane helix of interest within the membrane environment results in dimerization of the DNA-binding domain in the bacterial cytoplasm, and the dimeric DNA-binding domain then binds to the DNA and activates a reporter gene. Subsequently, systems with improved features, such as the TOXCAT- or POSSYCCAT system, which allow screening of TM domain libraries, or the GALLEX system, which allows measuring heterotypic interactions of TM helices, have been developed and successfully applied. Here we briefly introduce the currently most applied systems and discuss their advantages together with their limitations.