Understanding an ancient universal membrane effector system

All living cells are surrounded by a thin membrane, which keeps the inside of the cell separate from the outside environment. This essential cellular boundary layer contains proteins that allow the cell to take up food and expel waste products. The membrane is also energised, which means that cells actively generate and maintain ion gradients and voltage across the membrane. This so-called electrochemical gradient is one of the key cellular methods to store energy, and which can be utilised to drive uptake of nutrients and expel waste products. Furthermore, electrochemical gradient is critical for the synthesis of ATP, which is a key molecule used to energise intracellular processes. Hence, the integrity of the membrane is central to the proper function of living cells, and many known toxins including some antibiotics act by disrupting the membrane. During the early evolution of cells, a protein named IM30 evolved that has a role in protecting the membrane in the presence of damaging agents. These proteins, discovered just over 30 year ago, have been found to adopt ring-like structures that can stack upon each other to form tubes. Further, these proteins are known to accumulate in cells to very high levels when their membrane is damaged, and directly bind to cell membranes. However, how they protect the membrane, allowing the electrochemical potential to be maintained, is unknown.

In this project we have assembled a diverse team of researchers at different stages in their career and with different sets of expertise. We all share the interest in trying to figure out how the IM30 proteins work to protect cellular membranes. They could potentially do this by forming a coat covering the inside of the membrane, by forming 'ribs' that wrap around the cell and hold the membrane together, or by allowing the membranes to form small fragments that carry away the toxin that is damaging the cell. Alternatively, they might specifically project the proteins that generate the electrochemical potential or use it make ATP. Currently, we simply do not know. Whatever the mechanism is, however, it will be a completely new one and tell us important fundamental information about how biological membranes are organised and function.

To figure out how these proteins function, we will use a large number of different microbial species, as these represent relatively simple cells that we can study effectively, and where we know IM30s are important. We will systematically characterise how much of the protein different cells contain, where in the cells the proteins a localised, whether they assemble into rings and rods inside the cell, and how their function is regulated by other factors. We will develop new techniques to measure the electrochemical gradient in real time in living cells, which will provide us with essential tools to study the mechanisms through which IM30 proteins protect cell membranes. Our team comprises of microbiologists, biophysicist, biochemists, geneticists and cell biologists at 5 different Universities in the UK, who will come together to bring about a step change in understanding of IM30 protein function, and more generally how cells protect themselves from environmental insult.

The function of a cell membrane, which enabled maintenance of intracellular conditions that differed from the environment, was integral to the emergence of cellular life. These primordial membranes became ion-tight and able to sustain electrochemical gradients, which in turn allowed evolution of oxidative phosphorylation. The critical role of a 'sealed membrane' to the proper cell function also led to the evolution of mechanisms to protect it from damage, and to maintain membrane potential in the presence of damaging stresses. There is evidence for a cellular repair system that can 'reseal' membranes to restore membrane potential, which we have discovered is frequently activated in microbes producing chemicals for biotechnology and also by bacteria exposed to antibiotics. However, we know very little about how this fascinating membrane-protective system works. The key protein is a member of an ancient family called PspA/VIPP1 (IM30) proteins, first recognised in bacteria as PspA, which is present in cyanobacteria and plants as VIPP1 and which has recently been recognised to be structurally analogous to mammalian ESCRT-III proteins. Whilst the regulation of expression is understood in some bacteria and their propensity to form oligomeric rings in vitro is known, how they 'rescue' membrane function remains unknown. In this sLOLA we bring together a multidisciplinary team to understand how microbial IM30 proteins function at cellular membranes. Our approach combines microbial physiology, cell biology, genetics and metabolism, with biophysical, biochemical and proteomic methods to study the interaction of IM30 proteins with membranes and with their receptor proteins. This knowledge will help us understand how microbials have responded to membrane damaging stresses over millions of years, and provide insight into mechanisms of antimicrobial resistance (AMR) as well as a route to engineer microbes to produce compounds that are normally toxic to themselves.