Intracellular variation and membrane binding requires high protein expression levels to maintain the fidelity of cyanobacterial circadian rhythms in clonal populations.

Abstract

Giant unilamellar vesicles (GUVs) can bridge the gap between complex in vivo cell studies and in vitro bulk studies which occur in volumes considerably larger (∼10∧10 times) than cellular volumes. We demonstrate the benefits of in vitro GUV studies by functionally reconstituting the core oscillator from circadian clock of the cyanobacterium, Synechococcus elongatus, and show we can introduce intercellular variation to the GUV system. Unlike in bulk experiments, we discovered the fidelity of the circadian rhythms were highly dependent on protein levels in the vesicles and the size of the vesicles. By quantifying the encapsulation of proteins into vesicles, we show that the intercellular variation seen in cells can be mimicked through our diffusive loading process for our population of vesicles. We also show KaiB can bind to vesicle membranes, matching reports of binding to cellular membranes in vivo. We built a computational model to show how the fidelity of the circadian rhythms is modulated by the cell-like variation in protein concentrations and the membrane binding of proteins. When cell-like variation in introduced, we show the core oscillators of the clock is unable to achieve near ∼100% fidelity seen in the cell, which show why the transcriptional feedback and other complementary systems are necessary along with the core oscillator. We also show how membrane binding of proteins causes the clock fidelity to be modulated by the vesicle radius, r, due to the free protein reduction that is a function of the surface area to volume ratio (3/r). This work shows the potential for in vitro GUVs studies to mimic the cellular environment while maintaining the benefits of an in vitro system.