Primitive synthetic cells get a taste for each other

Predator–prey interactions are central to almost all ecosystems on Earth today, and their emergence is a key feature of the development of life. Now, researchers have observed a similar interaction between two types of very basic synthetic protocell. The work does not provide a blueprint for how life could have developed, but does present insights into the emergence of lifelike behaviour.

Modern cells are sophisticated molecular machines. But in recent years, a community of biophysicists, materials chemists and others have constructed several different types of much simpler ‘protocells’ in search of the key features necessary for collective behaviour to emerge from chemistry. Each study has usually focused on one type of protocell, says Stephen Mann of the University of Bristol, UK. Real ecosystems, however, comprise multiple interacting organisms. In the new research, therefore, Mann and colleagues combined aqueous dispersions of two different protocells: negatively-charged proteinosomes – protocells encapsulated by a protein polymer – and positively charged coacervate microdroplets – membrane-free protocells produced by phase separation in a liquid.

The researchers incorporated protease K – an enzyme that breaks down proteins – into the coacervate microdroplets. When the proteinosomes and the microdroplets were electrostatically drawn together, the microdroplets broke down the proteinosomes’ protein coats and ‘consumed’ the cells. The contents of the proteinosomes – which, in different experiments, included the polysaccharide dextran, platinum nanoparticles and single-stranded DNA – were incorporated into the coacervate microdroplets, which went on to eat further proteinosomes, eventually wiping them out. This constitutes a primitive form of artificial predation.

 Mann stresses that, as the chemicals used were not present on the early Earth, the work does not present a prebiotically plausible pathway for the emergence of life, but that the work could have applications. Microcapsules like protocells can deliver drugs to specific parts of the body and release them in response to specific stimuli. Mann speculates that one could ’build several populations of microcapsules, which could then work collectively in sensing a component, which could then switch on a second population, which could then switch on a third population – so you would have built-in functionalities that are community dependent’. This, however, is far in the future. The researchers are now working to incorporate other protocells into their synthetic ecosystem to study more complex relationships.

Christine Keating of Penn State University, US, is impressed: ‘This paper shows how very simple physico-chemical properties can lead to complex “behaviours” in systems with two or more interacting protocells,’ she says. ‘Although the initial interaction between predatory coacervate protocells and prey proteinosomes is due simply to their opposite surface charges, the consequences of their interaction are impressive.’ She notes the significance, in origin of life studies, of the fact that the material from the proteinosomes was incorporated into the microdroplets: ‘This work suggests that when we think, “how did this protocell acquire multiple crucial functional components?”, we should be considering the possibility that it “ate” its neighbours,’ she says.