By Konstantinos Vavitsas
Membrane proteins are notoriously difficult to work with, and are especially challenging to overexpress in heterologous hosts. In a study recently published in ACS Synthetic Biology and led by Omics Engine member Georgios Skretas, Greek scientists demonstrated how to express enhanced amounts of properly-folded membrane proteins in the bacterium Escherichia coli.
A cell membrane is much more than just an enclosing lipid container. As the layer that separates the cell from its environment, it controls what molecules are able to enter or exit; it participates in cellular signaling; and interacts with other cells and viruses. All these functions cannot not take place without membrane proteins.
Membrane proteins are what the name says, proteins that are either incorporated or strongly associated with a cellular membrane. Compartments, such as the endoplasmic reticulum, Golgi apparatus, organelles, and, of course, the cell membrane, are protein-rich. These proteins are precisely targeted, they adopt a specific fold, and they interact with the membrane to fulfill their cellular role. As a result, they cannot fold and function properly in a different cellular environment or in other organisms.
The inability to express membrane proteins in fast-growing bacterial hosts, such as E. coli, is hindering the study of these very important molecules. Heterologous production of membrane proteins usually leads to low production and increased cellular toxicity. In a work published in 2017, Dimitra Gialama and her coworkers explored how this issue can be resolved. By co-expressing a membrane-bound DnaK co-chaperone or an RNAse inhibitor, the researchers showed that it is possible to express large amounts of heterologous membrane proteins in bacteria.
In this work, recently published in ACS Synthetic Biology, Michou and her collaborators took the aforementioned research one step further. The two protein-overexpressing E. coli strains – SuptoxD and SuptoxR – were tested in different conditions to achieve production optimization. The study evaluated different parameters, such as heterologous mRNA production levels and growth temperature. The strains produced a variety of membrane proteins, belonging to all three domains of life, with different sizes and folds. The proteins were shown to be active and folded properly.
Membrane proteins are crucial for life, important pharmacological targets, and can be modules of interesting synthetic biology applications (such as tunable biosensors). The two optimized strains, SuptoxD and SuptoxR, can be invaluable in pushing forward the research and industrial applications in this field, expressing more and more challenging membrane proteins.