Scientists from the Institute of Biochemistry and Molecular Biology and Collaborative Research Center 746 of the University of Freiburg have discovered a new mechanism which plays an essential role in the assembly and growth of mitochondria, the "power plants" of the cell.

These organelles make energy stored in food ready for use by the cell. The generators in the cellular power plants are biological membranes located inside the mitochondria. Even minute errors in the composition of the inner mitochondrial membrane can lead to severe metabolic derangements, which can have an especially negative impact on the energy-hungry muscle and nerve cells.

In order to function, the cellular generators depend on the support of numerous highly specialized membrane proteins in the inner mitochondrial membrane. For the most part, these proteins are synthesized outside of the organelles and then imported with the help of protein translocases. Fundamental processes like this follow the same principles in all organisms, from unicellular life forms to human beings. The scientists were thus able to use mitochondria from baker's yeast as a model system for their study, which has now been published in the journal Current Biology.

In investigating the insertion of a family of membrane proteins which is of great pharmacological interest, the so-called ABC transporters, the research team made the surprising discovery that some segments of the transporters are evidently initially skipped by the insertion machinery and transported completely over the membrane. "These errors in membrane insertion are then repaired by another translocase which is very old from an evolutionary perspective," says Maria Bohnert, doctoral student and Boehringer-Ingelheim Scholarship recipient. Thus, the scientists were able to demonstrate for the first time that at least two different protein translocases cooperate closely to insert proteins with complex structures into the inner mitochondrial membrane.

In clarifying this coupled mechanism of membrane insertion, project head Dr. Martin van der Laan and his team have solved a hotly debated scientific problem and made a major contribution to our understanding of the composition and functioning of cellular power plants. The findings may help scientists to throw light on the mechanisms of diseases caused by defects in the biogenesis of mitochondria.