Our SPOT contains two main applications. One is to use the SPOT as a reaction hub to regulate various biochemical reactions. We have demonstrated that if we integrated three enzymes which could synthesize β-carotene into SPOT system, the production of β-carotene in yeasts will be enhanced if phase separation happens. In the future, if we use the yeasts to manufacture β-carotene in a large scale, we can sequester the yeasts in fermenters to prevented recombinant yeasts or recombinant DNA from flowing out into the environment. The other is to use our SPOT as a sensor. We achieved that if there are ABA or Rapamycin molecules in the environment, phase separation would happen. If we want to use the sensor in reality, we can package the yeast cells into resins. It will help us to observe the SPOT formation by microscopy, and limit yeast escape. In both two reality scenes, escape of microbes is prohibited.
We believe that our work has reached the medal requirements of demonstration as we have confirmed that our synthetic organelles can be formed in vivo and deliver a range of functions both for engineering and research due to their amazing properties. The concrete demonstration of the whole platform is shown below. You can see more details of experiments and modeling in our Data Page and Modeling
Our basic system consists of two components of synthetic organelles. Either of them has a specific HOtag to form homo-oligomers. We expect that they are able to form synthetic organelles due to the principles of phase separation. To verify the feasibility of the design, we fused two fluorescence proteins with the two components of synthetic organelles (Figure1.a) so that we can observe the self-organization of components and the formation of granules under fluorescence microscope.
We used SUMO-SIM interaction module to build a spontaneous organelle. When two components are expressed in yeasts, granules with the two fluorescence proteins can be observed in vivo (Figure1.b).
Meanwhile, by rapamycin induced interaction module, FKBP-Frb, we have built an inducible organelle. We can see granules occurs in yeasts within minutes after adding the inducer.
（这里可能需要一张cartoon的设计图）
b, c fluorescence images of spontaneous organelles (SUMO-SIM based) and inducible synthetic organelles (FKBP-Frb based, after adding 10000 nM rapamycin)The formation of organelles has flexible but predictable properties and kinetics in different conditions
As the model predicts, the concentration of components and the interaction strength affect the kinetics of phase separation. First we controlled the expression levels of components by using several stable or inducible promoters and observe the system's behavior. We found that the formation of organelles happened in specific promoter combinations and can be controlled by inducible promoters. The analysis result does not only fit well with the simulation, but provides potential methods to control the organelles in applications.
Figure2 (a) Phase diagram of a phase separation system with three components(simulation). To fit our system, the x-axis and the y-axis stands for the two components in the granules. The asymmetry comes from the assumption that the two components have different interactions with water. (b) Fluorescence movies of different promoter combinations of FKBP-Frb mediated system after adding rapamycin. Only in specific combinations, synthetic organelles can be formed by phase separation. (c) The formation process of SUMO-SIM mediated synthetic organelles can be controlled by inducible promoters. While the expression of Tet07-SIM-mCherry-HoTag6 is induced by dox gradually, the granules will occur abruptly in some time.
The strength of interaction modules can be also controlled. In the rapamycin-induced organelle system, changing the concentration of rapamycin will affect the apparent value of K, a parameter reflecting the interaction strength in our model. In a gradient rapamycin-inducing experiment, the delay time from adding inducer to granules formation was found to be shorter when concentration of rapamycin increases. So we have confirmed the influence of two parameters in models and increased the flexibility of our synthetic organelles.
Figure3 (a) A simulation of organelle formation process in different interaction strength of components. (b) The speed of FKBP-Frb mediated organelle formation increases with the increasing concentration of rapamycin.
Since SPOT can form in the cell and be controlled, we go further to consider the functions of SPOT. The functions of SPOT can be descripted in three catalogs: Spatial segmentation, Sensor and metabolic regulation. We verified the spatial segmentation with the condensation of substrates, also we can load the protein we want by fusing it with nanobody. We then verified the sensor with detecting rapamycin and ABA, which shows strong relativity between the concentration and the proportion of yeasts with SPOT. To find the law behind metabolism in the SPOT, we fuse the enzymes that can produce β-carotene into SPOT and measure the difference between with or without SPOT in produce of β-carotene.
SPOT has been well verified and has various functions. And in the future, this modular system will have great potential in science and practice using. SPOT can change the modules to gain more different properties like diverse inducing method, we can also use it as a platform and then load other protein with some interactions like the interaction between nanobody and GFP. What’s more, we might have the ability to form differernt SPOTs in the cell and regulate them respectively. The functions of SPOT can also diverse. We can build a real time sensor for molecule in living cells to monitoring the concentration changing in environment or in cells. More metabolism pathway can be test in SPOT and we will find some laws of the function of regulate the metabolism. To be summary, more achievement is coming true with SPOT.