The cellular protein machinery is a marvel of nature and produces umpteen different proteins. Most of these proteins pass through a stack of membrane-enclosed discs, known as the Golgi apparatus, a miniature reaction chamber where the finishing touch is progressively added to the proteins. Rudolf Hausmann, professor and head of the Department of Bioprocess Engineering at the University of Hohenheim, is developing membranes based on the Golgi model to facilitate production of proteins outside the cells.
Bioprocess engineers still use genetically modified bacterial or mammalian cells to produce proteins of interest in giant bioreactors. These proteins are used as drugs or enzymes in washing powder, to name but two examples. It is difficult to produce proteins that are toxic for the cellular metabolism or that do not exist in nature using this method. However, researchers are able to produce small amounts of such proteins outside cells using isolated cell organelles.
That said, engineers often want to chemically alter industrially produced proteins. For example, many pharmaceutical proteins are linked with polyethylene glycol, which surrounds the drug completely, thus protecting it from premature degradation in the body. "We have to remove the product from the reaction chamber after each chemical reaction step, and prepare it for the subsequent step," says Rudolf Hausmann, indicating just how laborious the procedure is. "We want to find an alternative to these step-by-step syntheses," he adds.
To transform proteins from unfinished to finished product in a series of uninterrupted steps in a bioreactor, the bioreactor needs to be divided into compartments just like a Golgi apparatus. Researchers also have to ensure that only proteins with certain signal sequences can pass through the membrane of the Golgi apparatus," says Hausmann. However, technically speaking, this is still a vision of the future.
"The problem is that artificial membranes cannot selectively separate proteins or other substances. They can only separate them according to size," says Hausmann. The bioprocess engineer and his team are specifically focused on the first step of this process, i.e. the production and entry of cell-free protein precursors into the artificial Golgi apparatus where they are chemically modified.
Hausmann's colleagues Ramona Bosch and Karin Moß produce so-called nanodiscs that are around ten nanometres in diameter, i.e. a 100.000th part of a millimetre. These biomembrane-like discs consist of a lipid bilayer held together by a ring-shaped protein. This protein is a genetically modified apolipoprotein that binds water-insoluble lipids such as cholesterol to form lipoproteins and transport them through the blood. They shield the fat-soluble part of the membrane fragment against the aqueous environment, thereby stabilising it.
The researchers plan to embed a protein transport channel into this construct. The resulting protein would then be transported through the transport channel into the next compartment while retaining the protein machinery and all basic components. "The current difficulty is finding a way to arrange the protein rings so that the nanodisc membrane forms a closed area," says Hausmann who is part of SeleKomM (Selective Compartment Membranes), a cooperative project that also involves researchers from the Universities of Ulm and Stuttgart as well as the Karlsruhe Institute of Technology.
Prof. Dr. Kay Gottschalk, Dr. Frank Rosenau, Prof. Dr. Tanja Weil and apl. Prof. Dr. Ulrich Ziener from the University of Ulm are tasked with embedding these nanodiscs into sheet-like plastic membranes. Prof. Dr. Martin Siemann-Herzberg from the University of Stuttgart is responsible for equipping the protein transport channels in the artificial membranes with a special charging system that produces energy in the form of ATP. Siemann-Herzberg also deals with cell-free protein production. Matthias Franzreb from the Karlsruhe Institute of Technology ensures that magnetic nanoparticles that bind to the proteins of interest are transported from one reaction compartment to the other when a magnetic field is applied. Together, the researchers are seeking to produce a biotech copy of the Golgi apparatus, and they will carry on working until they achieve this.