Topology, dimerization, and stability of the single-span membrane protein CadC

Under acid stress, Escherichia coli induce expression of CadA (lysine decarboxylase) and CadB (lysine/cadaverine antiporter) in a lysine-rich environment. The ToxR-like transcriptional activator CadC controls expression of the cadBA operon. Using a novel signal peptidase I (SPase I) cleavage assay, we show that CadC is a type II single-span membrane protein (S-SMP) with a cytoplasmic DNA-binding domain and a periplasmic sensor domain. We further show that, as long assumed, dimerization of the sensor domain is required for activating the cadBA operon. We prove this using a chimera in which the periplasmic domain of RodZ-a type II membrane protein involved in the maintenance of the rod shape of E. coli-replaces the CadC sensor domain. Because the RodZ periplasmic domain cannot dimerize, the chimera cannot activate the operon. However, replacement of the transmembrane (TM) domain of the chimera with the glycophorin A TM domain causes intramembrane dimerization and consequently operon activation. Using a low-expression protocol that eliminates extraneous TM helix dimerization signals arising from protein over-expression, we enhanced dramatically the dynamic range of the β-galactosidase assay for cadBA activation. Consequently, the strength of the intramembrane dimerization of the glycophorin A domain could be compared quantitatively with the strength of the much stronger periplasmic dimerization of CadC. For the signal peptidase assay, we inserted an SPase I cleavage site (AAA or AQA) at the periplasmic end of the TM helix. Cleavage occurred with high efficiency for all TM and periplasmic domains tested, thus eliminating the need for the cumbersome spheroplast-proteinase K method for topology determinations.