Benjamin L. Springstein: A key finding of our study is that bacterial polymer-forming protein systems are remarkably evolutionarily plastic and can be functionally repurposed to support entirely new cellular roles. We show that an ancestral DNA-segregation machinery was co-opted into a novel cytoskeletal system that controls cell shape in multicellular cyanobacteria, an event that coincided with the emergence of cyanobacterial multicellularity. This suggests that the evolution of bacterial multicellularity was accompanied by the acquisition of new structural functions from pre-existing molecular components. More broadly, our work demonstrates that seemingly conserved polymeric protein assemblies can be repeatedly adapted during evolution, implying that many additional examples of such functional innovation remain to be discovered across the tree of life.
Martin Loose: One of the most exciting insights from our paper is seeing evolution in action, not as an inventor, but as a tinkerer.
We found that cells have taken two emergent properties of protein systems that already existed for completely different tasks and repurposed them in a remarkably creative way. The first is dynamic instability: the ability of protein filaments to rapidly grow and collapse. This property is normally used by ParM to form plasmid-segregating filaments to push DNA to opposite ends of a cell before division. In multicellular cyanobacteria, we found that this same filament system, that we renamed to CorMR, has been evolutionarily repurposed to instead control cell shape via a cortical cytoskeleton.
The second is protein oscillations: the Min system, which oscillates from pole to pole in bacteria to mark the cell's midpoint, controlling where FtsZ polymerizes and where division occurs. We discovered that in these cyanobacteria, the Min system has acquired an additional role, it also regulates CorMR polymerization, controlling where these filaments can grow.
What makes this so interesting is that it shows that evolution didn't invent anything new from scratch. Instead, it coopted two pre-existing behaviors, dynamic instability and proteins oscillations, and wired them together for an entirely new purpose: building and maintaining the shape of multicellular cyanobacterial cells. I think this is a beautiful example of evolutionary tinkering, and that the possible functional repertoire of these protein systems is far broader than we previously recognized.
