Mechanisms of Signaling and Regulation of Membrane Properties by a Bacterial Thermosensor

DE MENDOZA, DIEGO

Boston

Simposio; Biophysical Society; 2009

The ability of bacteria to control the biophysical properties of their membrane phospholipids allows them to thrive in a wide range of physical environments. When bacteria are exposed to temperatures below those of their normal conditions, the lipids of their membrane become rigidified, leading to a suboptimal functioning of cellular activities. These organisms can acclimate to such new conditions by an increase in the desaturation of the acyl chain of membrane phospholipids. Phospholipids containing unsaturated fatty acids have a much lower transition temperature than those lipids made of saturated fatty acids. As a result, the physical properties (fluidity) of the membrane lipids return to their original state, or close to it, with restoration of normal cell activity at the lower temperature. We discovered that in the model Gram-positive bacterium Bacillus subtilis the transcription of the gen des, coding for an acyl lipid desaturase, is controlled by a two component system that senses changes in the membrane properties due to abrupt temperature change. The membrane component, named DesK, of this transcriptionally regulatory system is a bifunctional histidin kinase/phosphatase that senses membrane biophysical properties and transmits this signal to the transcriptional apparatus. We undertook structural and in vitro reconstitution studies of DesK to gain further insight into the outstanding question of how does this membrane viscosity sensor operate at a molecular level. The crystal structures of the cytosolic domain of DesK reveal  ground states seemingly poised for autokinase phosphotransfer or phosphatase activity, and reinforces the notion of multiple conformational states of the enzyme along the catalytic cycle. In addition, we reconstituted purified full-length DesK into liposomes and determined its different catalytic activities. Structural and biochemical-inspired hypothesis for the distinct catalytic mechanisms and for signal transduction through the membrane to the cytoplasmic domain will be discussed.