A question of fundamental importance in biology is how order spontaneously emerges from homogenously distributed building blocks, for example in the formation of cells and the many biochemical processes occurring within them to spatially organise processes such as motility and division. This transformation from a homogenous state to one where components are spatially organised is often referred to as ‘symmetry breaking’. One process by which it is achieved is called self-organisation. One of the most fundamental biological self-organisation processes is cellular polarisation, for example as extensively studied in budding yeast, whereby a cell spontaneously orients molecules within it such that a single focus on the cell membrane is formed. This polarization process underlies both the mating behaviour of yeast as well as cell division, where daughter cells form and bud off from this focal point.
Some theoretical models suggest that cellular polarisation can spontaneously occur through a simple mechanism that requires: (i) reversible lipid membrane binding, (ii) diffusion while bound upon the membrane, and (iii) positive feedback, i.e. membrane-bound molecules recruit more molecules to bind the membrane. The goal of this project was to test these models by building a synthetic system from the bottom up that recapitulated these key features. As well as helping us understand the mechanism by which symmetry breaking, such as in cellular polarisation, occurs, achieving this goal would also provide useful tools for the creation of ‘proto-cells’—artificial, self-contained systems that exhibit life-like processes. Our strategy incorporated a key tenet of synthetic biology, namely that the system should be built in a modular manner from minimal components. In this case, the minimal modules were to be minimal peptides and protein domains. The project contained two planned milestones: first, to construct a reversible membrane-binding switch; and second, to subsequently add additional modules to generate positive feedback, and thereby generate symmetry breaking.
