Impact of the substrate on the phase transitions properties in supported lipid bilayers

Bacterial membranes are self-assembled structures, comprising a phospholipid bilayer embedding proteins and sugars. It plays a crucial role in the survival of the organism, from providing a physical barrier with the surroundings, to controlling membrane trafficking,and underpinning drugs resistance. The activity of the proteins and their response to external stimuli depends on the biophysical properties of the bilayer such as its thickness, elasticity, packing and molecular mobility. Often overlooked is the fact that membranes are always in close contact with various structural filaments such as cytoskeleton tubules or peptidoglycan chains which can all influence the properties of the bilayer.

Here, we employed a minimal bi-component model system of E. coli’s inner membrane to study the effects of these external interactions on the nanoscale phase behaviour of the model membrane. Using a combination of atomic force microscopy and differential scanning calorimetry, we comparatively track the phase transition kinetics, comparing situation when the membrane is supported and unsupported. The results show that only part of the kinetics depends on the cooling/heating, with the main influence coming from contact with a support. The presence of a contacting substrates not only shifts the transition temperature, but it can also arrest the transition and induce a global re-arrangement of the lipid species. This results in a phase transition that partially follows classical nucleation on short timescales but moves on to a spinodal decomposition on the longer term.

Our work highlights the importance of external membrane contacts to control the composition and biophysical properties of bacterial membranes.