EnvZ signal output correlates with capacity for transmembrane water encapsulation

In Gram-negative bacteria, porins span the outer membrane and control the influx of several prominent groups of antibiotics. Expression of these porins is often altered in clinical isolates exhibiting multidrug resistance. The major regulator of porin expression in Escherichia coli is EnvZ, a canonical sensor histidine kinase. It allosterically processes periplasmic interactions with MzrA and cytoplasmic osmosensing into a single unified change in the ratio of its kinase and phosphatase activities. To better understand how transmembrane communication is achieved by EnvZ, we began by performing disulfide crosslinking to monitor the position and dynamics of helices within the transmembrane domain in vivo and correlated these with modulation of signal output. Subsequently, we employed atomistic molecular dynamic simulations on full-length EnvZ embedded in a model Gram-negative membrane and observed that this in silico analysis accurately predicted the structure of the transmembrane domain embedded in the bacterial membrane. We conclude by proposing that the signal output of EnvZ correlates with its potential for water encapsulation within the transmembrane helical bundle and with unwinding of a helix adjacent to the periplasmic end of the second transmembrane helix. When combined with results from other bacterial receptors, it remains plausible that water encapsulation occurs in other topologically similar sensor kinases.