Mitochondria contain an intertwined membrane system that is necessary for the production of energy. Errors in the inner mitochondrial membrane architecture prevent energy from being produced, which can result in disorders of the muscular or nervous system. A group of researchers led by Dr. Martin van der Laan at the University of Freiburg, in cooperation with other partners, has identified a novel protein complex that plays a key role in the architecture and functioning of the mitochondria. Further research has provided the researchers with evidence of the key role that the huge molecular machine plays in importing new proteins into the mitochondria and bringing together key molecular interaction partners at the two mitochondrial membranes.
The mitochondria and their labyrinthine membrane system, within whose walls the entire cell respiration machine can be found, resemble a country of mountains and wide valleys when looked at under the electron microscope. Mitochondria are specialised organelles in cells that generate most of the cell's supply of energy. They can only generate this energy if the cristae, which are invaginations of the inner mitochondrial membrane, have a characteristic morphology, which makes room for microenvironments where proton gradients can build up, for example. These protein gradients generate an electrochemical potential and hence energy. "The cristae have a very complex structure; depending on which tissue they are located in, they can have many different forms, some of them can look rather like artistic sculptures. Errors in this complex structure can lead to disorders in the energy-dependent tissues such as skeletal muscle and the nervous system," said Dr. Martin van der Laan, who is head of a group of researchers at the Institute of Biochemistry and Molecular Biology at the University of Freiburg and a member of the collaborative research centre 746 (SFB, Functional specificity by coupling and modification of proteins). An important issue, for both science and medicine, is the clarification of how the architecture of the cristae is generated and regulated.
Van der Laan and his team were originally interested in finding an answer to the question as to how newly synthesised proteins are transported into the mitochondria. Mitochondria have their own genome, which is why they are believed to originate from bacteria that were taken up by a predecessor of modern eukaryotic cells about 1.5 billion years ago. Although they have their own genome, they only produce a small number of the proteins they need and import the rest from elsewhere. Some years ago, van der Laan and his group of researchers managed to establish an artificial mitochondrial membrane system in test tubes using known constituents such as lipids and protein transport machines and they also identified the key constituents in the controlled import of proteins. The researchers also focused on how the characteristic morphology of the inner mitochondrial membrane is generated. It is known to consist of an inner boundary membrane and large tubular invaginations termed cristae. It is further known that the crista membrane and the inner boundary membrane are connected by crista junctions; in addition, the degree of opening of the cristae is regulated by a protein or protein complex about which very little is known. “We therefore focused on finding out more about the function of this protein complex at crista junctions,” said van der Laan.
Van der Laan's researchers carried out a comprehensive literature search. They also carried out numerous experiments in cooperation with a group of researchers led by Prof. Dr. Bettina Warscheid from the Centre for Biological Signalling Studies (BIOSS) in Freiburg. Using the yeast Saccharomyces cerevisiae as model organism, the scientists identified a huge protein complex at the crista junctions that is key in creating and maintaining the characteristic morphology of the inner mitochondrial membrane. When the scientists silenced certain components of this complex, the inner membrane morphologically altered dramatically with sheet-like cristae becoming detached from the inner boundary membrane. Van der Laan and his team have since found out that the complex consists of at least six proteins. A protein of the mitofilin family, which is found in all organisms, was identified as a key component of the complex. "Prior to our study, very little was known about how the characteristic morphology of the inner mitochondrial membrane system is generated and maintained," said van der Laan. "We were really pleased to have made this discovery." The majority of the study, which was recently published in the renowned journal "Developmental Cell", was carried out by van der Laan's colleagues Karina von der Malsburg, Maria Bohnert and Judith Müller. The three researchers also came up with the idea of naming the newly discovered protein complex MINOS during work which involved the analysis of electron microscope images of mitochondria. MINOS is the acronym of mitochondrial inner membrane organizing system; it is also the name of an ancient Cretan king who built the legendary labyrinth of Knossos whose representation on ancient coins looks rather like mitochondria seen under the electron microscope.
The MINOS complex is necessary for crista membranes to remain connected to the inner boundary membrane. The researchers also found that it mediates the connection of the inner and outer mitochondrial membranes. Van der Laan and his team have identified numerous interaction sites (contact sites) between the MINOS complex, which is located on the inner membrane, and the outer membrane. The first contact site identified connects the inner membrane with a protein transporter in the outer membrane (TOM) and with a protein in the intermembrane space (Mia40) that has a key role in oxidative protein folding. This connection is essential for the transport of proteins into the intermembrane space. Nothing is known about the function of other contact sites and further research needs to be carried out in this area. Van der Laan's research group has come up with the following working hypothesis: "We believe that MINOS is a component of a molecular structure that is most likely much bigger and probably forms a kind of protein skeleton in the mitochondria. We assume that the contact sites between the individual components help to bring the correct interaction partners together so that they can jointly carry out key functions in the mitochondria."
As the mitochondria are the major producers of cellular energy, changes in their characteristic morphology might have drastic effects in most tissues. The organelles are also involved in processes such as apoptosis and cellular ageing. Van der Laan and his team plan to work with Freiburg research groups who are interested in aspects of clinical relevance, for example the role of mitochondria in diabetes, cancer or progressive muscular and nervous diseases. "Our findings are opening up a completely new field of research," said van der Laan who believes that research into signalling processes will also benefit from their findings. There is increasing evidence that all the membrane systems of a cell - including the outer plasma membrane, the membranes of mitochondria, cell nuclei and endoplasmic reticulum - are connected with each other. It is therefore assumed that the membranes are the carriers of signals that spread by way of protein complexes such as MINOS, rather than by way of the reversible modification of cellular molecules and free diffusion in the cells. "Of course, we do not know for sure. This is our vision for the future and we will have to do many more experiments before we are able to come up with further details," said van der Laan. However, this vision reflects the huge enthusiasm that the researchers have for their project as well as showing how research works: a new finding always leads to many new perspectives and ideas that drive research forward.