GE Healthcare and the journal Science hosted an essay contest recently for young life scientists, and published the winning essay last week online. Written by Irene Chen, it dealt with the emergence of cells during early life – specifically, the merger of membranes and genetic material. Chen writes:
The combination of a genome and membrane does not constitute a unified cell unless interactions between the components result in mutual benefit. Was it a lucky accident that genomes and membranes began to cooperate with each other (e.g., evolution of an enzyme to synthesize membrane lipids)? Or are there simple physicochemical mechanisms that promote interactions between any genome and membrane, leading to the emergence of cellular behaviors? We explored such mechanisms experimentally, using model protocells.
To support this model, Chen cited a few lines of experimental evidence:
- She found that “The hammerhead ribozyme, which catalyzes a self-cleavage (or ligation) reaction, is active when encapsulated in vesicles composed of fatty acid (myristoleic acid) and its cognate glycerol monoester”
- And that “Vesicles encapsulating high concentrations of RNA experienced substantial osmotic stress, driving the uptake of fatty acid from unstressed membranes. This resulted in the transfer of ~25% of the membrane from empty vesicles to vesicles containing RNA, relieving the membrane tension caused by the osmotic gradient.”
- Suggesting that “As soon as replicators became encapsulated, a primitive form of competition could emerge between cells (see the figure). Remarkably, this process does not require a chance increase in complexity (e.g., addition of a new enzyme), but instead relies only on the physical properties of a semipermeable membrane encapsulating solute.”
That’s a phenomenal experiment. Sure, it’s only hypothetical with regards to how cells actually emerged billions of years ago, but the proof of concept is there. But Chen doesn’t stop there – “In a complementary experiment, we also demonstrated how membrane fitness (i.e., growth) might contribute to cellular fitness.”
- “We found that membrane growth generated a transmembrane pH gradient, due to the faster flip-flop of protonated fatty acid molecules incorporated into the outer leaflet of the membrane… In modern biological systems, pH gradients are widely used for energy storage and transduction. For a protocell, this energy might even be directly useful for driving cellular processes, such as the uptake of amines to aid RNA folding. Again, no additional enzymes need to be evolved for this basic form of energy capture and storage, which is only a consequence of the physical properties of the vesicles.”
- “…a corollary of vesicle competition is that a charged genetic polymer, such as nucleic acid, would be much more effective at driving membrane uptake than an electrically neutral polymer, because most of the osmotic pressure is due to counterions associated with the charged polymer. Could this influence the natural selection of the genetic material itself?”
- “Furthermore, competition for membrane molecules would favor stabilized membranes, suggesting a selective advantage for the evolution of cross-linked fatty acids (e.g., di- and triglycerides) and even the phospholipids of today.”
- “Greater membrane stability leads to decreased dynamics, however, and the evolutionary solutions to this problem (e.g., permeases, synthetic enzymes) could cause a “snowball” effect on the complexity of early life”
Chen nicely cited all of the work that went into these discoveries as well – work that will of course need to be followed up, verified, and elaborated upon.
Nice writing – I can’t believe I haven’t heard commentary in the science blogosphere on this yet!!!