Engineering a Thermosensor To Dissect a Transmembrane Signaling System

Abstract

The DesK-DesR two-component system regulates the order of membrane lipids in the bacterium Bacillus subtilis by controlling the expression of the des gene coding for the delta 5-acyl-lipid desaturase. In this work, we address the process by which DesK transmembrane segments (TMS) transmit temperature signals across the membrane by engineering the 5 TMS domain of the DesK into a single-TMS chimeric sensor. This so-called Minimal Sensor (MS) fully retains in vivo and in vitro the sensing input and transmission output of the parental system. Progressive deletions of TM segments revealed that only the first TM segment (TM1) is essential to regulate the kinase activity. Therefore, our engineered MS combines the N-terminal 17-residue portion of TM1 with the C-terminal 14-residue portion of TM5 which is naturally fused to the cytosolic catalytic domain. The MS N-terminus contains three hydrophilic aminoacids near the lipid-water interface creating an instability hot spot. This boundary-sensitive motif controls the sensing and transmission activity. Accordingly, we hypothesize that membrane thickness is the temperature agent that determines the signaling state of the cold sensor by dictating the hydration level of the meta-stable hydrophilic spot. This hypothesis is supported through the study of the signaling behavior of MS variants purposely constructed.