Synthetic biology seeks to build artificial cells embodying life-like qualities using minimal components. Achieving self-propelled motion remains a formidable challenge in this endeavor. However, spearheaded by Erwin Frey, Professor of Statistical and Biological Physics at LMU, and Petra Schwille from the Max Planck Institute of Biochemistry, a team of scientists have made significant strides in this field, as discussed in their recent publication in Nature Physics.

The researchers successfully perpetuated continuous movement of liposomes—vesicles encapsulated by a lipid membrane—on a supporting membrane. This motion results from the interaction between the vesicle membrane and specific protein patterns, sustained by the biochemical "fuel" ATP. These patterns are derived from the Min protein system, a known biological pattern formation system in charge of cell division in E. coli bacteria. Schwille's laboratory experiments indicated that Min proteins in the artificial system asymmetrically align around the vesicles and interact to induce their motion. The proteins bind to both the vesicles and the supporting membrane. Schwille notes, “The directed transport of large membrane vesicles is only found in higher cells, where complex motor proteins perform this task. To discover that small bacterial proteins are capable of something similar was a complete surprise. It is currently unclear not only what the protein molecules do at the membrane surface, but also for what purpose bacteria could need such a function.”

“We achieve persistent motion of cell-sized liposomes. These small artificial vesicles are driven by a direct mechanochemical feedback loop between the MinD and MinE protein systems of Escherichia coli and the liposome membrane,” the study authors wrote. “Membrane-binding Min proteins self-organize asymmetrically around the liposomes, which results in shape deformation and generates a mechanical force gradient leading to motion.”

The authors went on to explain that “the protein distribution responds to the deformed liposome shape through the inherent geometry sensitivity of the reaction–diffusion dynamics of the Min proteins. We show that such a mechanochemical feedback loop between liposome and Min proteins is sufficient to drive continuous motion.”

Frey's group used theoretical analysis to identify two potential mechanisms behind the observed motion. Frey elaborates, “One possible mechanism is that the proteins on the supporting membrane interact with those on the vesicle surface somewhat like a zipper and form or dissolve molecular compounds in this way. If there are more proteins on one side than on the other, the zipper opens there while closing on the other. The vesicle thus moves in the direction in which there are fewer proteins.” The other mechanism involves the proteins attached to the membrane altering the vesicle membrane's curvature, which drives its movement.

Frey asserts, “Both mechanisms are possible in principle. What we do know for certain, however, is that the protein patterns on the supporting membrane and on the vesicle cause the motion. This represents a big step forward on the road to artificial cells.” The team firmly believes their system will serve as a blueprint for designing artificial systems that mimic life-like movements in the future.

“Our combined experimental and theoretical study provides a starting point for the future design of motility features in artificial cells,” the authors concluded.

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