Towards a comprehensive model of plant plasma membrane lipid bilayer – PLAYMOBIL

In all organisms, the plasma membrane (PM) forms a selective barrier between the cell and the extracellular medium. It is a sensor for modifications of cellular environment, and a platform orchestrating signal transduction allowing translation of external signals in a finely tuned appropriate responses. The cell PM thus plays across kingdoms a critical role in cell physiology. The universal basic PM structure, established from the fluid mosaic membrane model (FMMM), is a lipid bilayer, in which proteins are embedded or associated to via a variety of interactions. Data accumulated since the publication of the FMMM revealed the unexpected and outstanding complexity of PM organization, and the essential role of lipids. Major classes of lipids are shared by all living organisms, but plants exhibit some striking characteristics including the different molecular species of sterols, free or conjugated, or specific sphingolipids. The heterogeneity of PM and the relationships between the dynamics of membrane organization and cell signalling has recently emerged as a key feature of cell biology. In plants, the presence of nano- to micro-scale domains exhibiting different lipid and protein content and biophysical characteristics, was demonstrated, together with the differential ability of plant lipids to generate such biophysical heterogeneity. Moreover dynamic relocalization of lipids and proteins within PM, concomittent with modifications of PM order and fluidity, have been particularly documented in early steps of plant-microorganism interactions. Deciphering the basis of PM organization at the molecular scale thus appears as a key step to understand the ability of plant cells to face the different environmental stress. In this context, a crucial point to describe PM organization is the asymmetry between the PM leaflets, and the remaining unraveled mechanisms of their coupling : we need to understand how the PM is able to ensure its physiological function, coordinating the translation of signals coming from the cell medium in which the outer leaflet is exposed, into appropriate responses through the activation of the cellular machinery facing the inner leaflet. We are currently missing reliable and comprehensive data describing plant PM asymmetry, regarding both its composition and organization. In this project, we will take advantage of the very complementary expertise of the different partners in membrane biochemistry, lipid analysis, cell signaling and modelling to develop an interdisciplinary project addressing this crucial question. The project will use the model plant Arabidopsis thaliana to : 1/ produce the full reference lipidome for the plant PM; 2/ address the question of plant PM asymmetry through the characterization of the lipid composition and in vivo biophysical characteristics of each PM leaflet 3/ combine atomistic and coarse-grained modelling approaches, used in back and forth with data generated by these experiments to provide an unprecedented comprehensive model of the plant PM bilayer. Genetic and pharmacological tools will allow to test in vivo the influence of the different lipids identified (i) on the biophysical characteristics of the inner and outer leaflets, (ii) on the dynamics of proteins differentially anchored to the PM inner and outer leaflets, (iii) on the very early steps of immune signaling. Our project will benefit from state of art methodologies to produce substantial cutting edge results, and also put a strong focus on the production of data which should help the plant community to move a step forward on these questions. Combining results of experimental and modelling approaches the project aims at establishing the foundations for a global understanding of plant PM leaflet asymmetry regarding lipids, which are essential constituents providing the core architecture and instrinsic biophysical features of PM, and shedding light on the wide open question of the molecular coupling between two leaflets.