How are the cell membranes organized

How proteins are stored in a cell membrane

Many proteins with important biological functions are embedded in a biomembrane in the cells of humans and other living things. How do you even get in there? Researchers at the Department of Biosystems Science and Engineering at ETH Zurich have investigated this.

Almost a third of all proteins in living beings are stuck in a biomembrane - either in the outer membrane of a cell or in the boundaries of cell-internal compartments. There these membrane proteins take on important tasks, for example as molecular locks that transport metabolic products and nutrients through the membrane, or as sensor proteins to record the cell environment.

How membrane proteins get into the membranes in the first place has now been investigated by researchers led by Daniel J. Müller, Professor at the Department of Biosystems Science and Engineering at ETH Zurich in Basel. To do this, they used a high-precision method with which they can pull individual proteins out of membranes or deposit them on membranes. Single-molecule force spectroscopy is the name of the method in which a computer-controlled leaf spring, which is only a few nanometers thick, can be precisely directed to a location on a membrane surface. Molecular adhesion forces ensure that a protein located there adheres to the leaf spring.

Role of two helper proteins

In experiments with bacterial proteins, the researchers were able to elucidate the role of two helper proteins that enable membrane proteins to insert themselves into the membrane: a so-called insertase and a translocase. The former is a protein, the latter a complex of several proteins. Both ensure that a pore opens in the membrane. “In the case of the insertase, this pore can be imagined as a slide. The membrane protein is initially available as an unstructured peptide thread that slides into the membrane on this slide. This peptide thread then organizes itself in the membrane into its functional three-dimensional shape, ”explains ETH Professor Müller. "Once the membrane protein has been successfully three-dimensionally shaped and anchored in the membrane, the helper protein detaches itself and forms a slide for the next protein elsewhere in the membrane."

How these helper proteins work has so far only been investigated imprecisely and only with very small protein fragments or outside of biomembranes. "For the first time, we have now observed and described step by step how an entire protein is inserted into a membrane and shaped in three dimensions," says Tetiana Serdiuk, postdoc in the group of ETH Professor Müller and first author of the study.

The ETH researchers were also able to demonstrate the different modes of operation of insertases and translocases: insertases insert peptide threads into the membrane relatively quickly, but in an uncoordinated manner. “So they do a good job, especially with small proteins,” says Müller. Translocases, on the other hand, insert peptide threads into the membrane section by section and are therefore more suitable for more complex proteins.

Central to medicine

This study is classical basic research, which is particularly important given the importance of membrane proteins for medicine, as Müller emphasizes: “Around half of all drugs act on membrane proteins, and we have to understand how these membrane proteins are formed and how they are formed function."

In addition, the single-molecule force spectroscopy, which the ETH scientists further developed for this study, could be used in additional applications: Within the framework of the national research focus “Molecular Systems Engineering”, Müller and other scientists are working on the development of artificial biological cells. “The method could be used to populate membrane envelopes with customized proteins and use them to“ program ”them,” says the ETH professor. "Such artificial cells could one day be used as molecular factories for the industrial production of medicinal substances."

Bibliography

Serdiuk T, Anja Steudle A, Mari SA, Manioglu S, Kaback HR, Kuhn, A, Müller DJ: Insertion and folding pathways of single membrane proteins guided by translocases and insertases. Science Advances 2019, 5: eaau6824, doi: 10.1126 / sciadv.aau6824