Membrane Thickness Cue for Cold Sensing in a Bacterium

Abstract

Thermosensors are ubiquitous integral membrane proteins found in all kinds of life. They are involved in many physiological roles, including membrane remodeling, chemotaxis, touch, and pain [1-3], but, the mechanism by which their transmembrane (TM) domains transmit temperature signals is largely unknown. The histidine kinase DesK from Bacillus subtilis is the paradigmatic example of a membrane-bound thermosensor suited to remodel membrane fluidity when the temperature drops below approximately 30°C [1, 4] providing, thus, a tractable system for investigating the mechanism of TM-mediated input-output control of thermal adaptation. Here we show that the multimembrane-spanning domain from DesK can be simplified into a chimerical single-membrane-spanning minimal sensor (MS) that fully retains, in vivo and in vitro, the sensing properties of the parental system. The MS N terminus contains three hydrophilic amino acids near the lipid-water interface creating an instability hot spot. Mutational analysis of this boundary-sensitive beacon revealed that membrane thickness controls the signaling state of the sensor by dictating the hydration level of the metastable hydrophilic spot. Guided by these results we biochemically demonstrated that the MS signal transmission activity is sensitive to bilayer thickness. Membrane thickness could be a general cue for sensing temperature in many organisms.

LPS remodeling is an evolved survival strategy for bacteria

“…For many bacterial pathogens, an important environmental stimulus is a shift in temperature from vector or environment to the human host. Temperature change has been demonstrated to induce bacterial membrane remodeling and is required for maintenance of optimal membrane architecture (3)(4)(5).…”

Activation of the bacterial thermosensor DesK involves a serine zipper dimerization motif that is modulated by bilayer thickness

“…Some evidence suggests that such changes in membrane thickness may be a key factor in the regulation of DesK sensing and signaling. First, an MS-DesK length mutant (4V) containing four extra valines in the C-terminal region of its transmembrane segment was found to be inactive and to remain locked in the phosphatase state on a decrease in temperature (5). Second, reconstitution studies showed increasing activity of both DesK and MS-DesK with increasing acyl chain length of the lipids in which the protein is reconstituted (5,8).…”

“…Importantly, MS-DesK shows a temperature-dependent switch in activity comparable to the full-length DesK not only in vivo, but also when reconstituted in protein-free lipid bilayers made from bacterial lipids (5). Therefore, no other membrane proteins are involved in sensing or signal transduction.…”

“…This challenge was recently simplified by the discovery that both the sensing and the signal transduction properties of the membrane-spanning part of DesK can be captured into a single transmembrane segment by fusing the Nterminal part of the first transmembrane segment to the C-terminal part of the fifth transmembrane segment (5). The resulting protein is called the minimal sensor DesK (MS-DesK).…”

“…Therefore, no other membrane proteins are involved in sensing or signal transduction. Furthermore, the activity of the catalytic domain DesKC itself, is not temperature-sensitive (5,6), and thus it must be the transmembrane segment of MS-DesK that somehow reacts to changes in temperature, most likely by sensing corresponding changes in the physical properties of the lipids.…”