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Membrane proteins and the import business of
mitochondria
The powerhouses of cells are surprisingly dependent on external help. More than ninety per
cent of all proteins required by the mitochondria are produced outside the outer
mitochondrial membrane. How are these proteins transported across the membrane and
how do they find their way into the mitochondria? A group of researchers led by Prof. Dr.
Chris Meisinger at the University of Freiburg has been investigating the role of large protein
complexes in the outer mitochondrial membrane for many years. The researchers have
discovered previously unknown protein import and sorting pathways. However, their
research has also provided evidence for the mitochondrias' important role in another
cellular process: they play a key role in cellular signalling, for example in programmed cell
death. It is assumed that membrane protein complexes also play a key role in this process.
Many diseases are the result of mitochondrial defects. Mitochondria are normally the
powerhouses of the cells: electrons are transferred from carbon compounds to oxygen in the
interior of mitochondria with the aid of a complicated system of protein complexes. This leads
to the release of energy. If something goes wrong during this process, combined disorders of
the energy-dependent tissues such as the skeletal muscles, the heart and the central nervous
system will typically arise. But the mitochondria are also associated with diseases that are not
related to the energy metabolism, for example the development of tumours or
neurodegenerative diseases such as Alzheimer’s and Parkinson ’s. There is growing evidence
that mitochondria also play a key role in cellular signalling, for example in apoptosis
(programmed cell death). Mitochondria trigger apoptotic processes when they are triggered by
external signals to do so. Therefore, the mitochondria are key in maintaining a well functioning
tissue complex that is free of abnormal or damaged cells.
A fertile comprehensive approach
"Although mitochondria contain their own DNA, only around one per cent of the proteins
required by Saccharomyces mitochondria for their work, can be produced by the mitochondria
themselves," said Prof. Dr. Chris Meisinger from the Institute of Biochemistry and Molecular
Biology at the University of Freiburg. "Thus, 99% of all required proteins are encoded in the cell
nucleus and need to be transported into the mitochondria." This is also the case for the
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Research group led by Prof. Dr. Chris Meisinger (left) at the Institute of Biochemistry and Molecular Biology at the
University of Freiburg. © Prof. Dr. Chris Meisinger
mitochondria in higher organisms, including humans. Huge molecular sluices in the
mitochondrial membranes, so-called protein import complexes, play a key role in importing
nuclear proteins into the mitochondria. The so-called TOM complex (transporter of the outer
membrane) is located in the outer of the two mitochondrial membranes. It recognises arriving
proteins that are allocated to the mitochondria and guides them to a channel in the
membrane. But what happens then? How do the proteins know whether they need to integrate
into the outer membrane, or whether they are destined for the intermembrane space, whether
their journey continues across the inner membrane into the so-called mitochondrial matrix?
“In order to gain detailed insights into the transport of proteins into mitochondria, we have
been pursuing a comprehensive approach,” said Meisinger going on to add “we are using
functional proteomics methods to do this.” Meisinger’s team and the research group led by Dr.
Albert Sickmann in Würzburg scanned the mitochondrial proteome a few years ago, i.e.
identified all proteins found in mitochondria in the model organism Saccharomyces. Of the 851
proteins identified, the function of one quarter is still unknown. Meisinger and his colleagues
then searched for promising candidates in this pool, especially those that were involved in the
import of proteins. The majority of proteins found in yeast mitochondria are not required by
the organism. When grown in a petri dish with a high enough amount of glucose, the
unicellular yeast simply switches to anaerobic energy production, which still works with a
minimal set of proteins. The majority of proteins involved in the import of proteins are
nevertheless essential, despite the minimal programme. “We have therefore been looking for
molecules without which the yeast is unable to survive in a glucose medium,” said Meisinger
going on to add “in the hope they had something to do with protein import.”
A view into primitive times
After many laborious biochemical and molecular biological experiments in which the
researchers systematically switched off genes suspected of being involved in the import of
proteins, they found several components of a large molecular machine located in the outer
mitochondrial membrane. This complex, which Meisinger and his team termed ‘sorting and
assembly machinery’ (SAM complex) after carrying out additional experiments, is closely
connected with the TOM complex. Proteins with a complicated structure that enter the
mitochondria by way of the TOM channel and are integrated into the outer mitochondrial
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membrane are guided to the SAM complex, which integrates them into the membrane and
ensures that they are embedded in the right steric conformation. One component of the SAM
complex is already found in the bacterial ancestors of the mitochondria. This clarifies the
evolutionary importance of the sorting machinery. “We even assume that SAM existed before
TOM, because the bacteria did not need to import proteins from the outside,” said Meisinger.
“This became necessary when the bacteria were taken inside eukaryotic cells as
endosymbionts, thereby giving rise to mitochondria.”
The two membranes of mitochondria and the protein import and sorting machinery acting in and around them. ©
Prof. Dr. Chris Meisinger
In addition to the SAM complex, Meisinger’s group of researchers, in cooperation with the
research group of Prof. Nikolaus Pfanner from the same institute, discovered the molecule
Mia40, a factor for the import of proteins into the intermembrane space of mitochondria.
Mia40 helps the proteins to fold correctly and assume the conformation that is necessary for
their correct function. The membrane-bound TOM and SAM protein import and sorting
complexes and sorting helpers such as Mia40 are indispensable for the mitochondria that need
to import the majority of required proteins from the outside. Mitochondria need the imported
proteins to produce the energy required by the cells. But the machinery located in the outer
mitochondrial membrane can do a lot more. “We have scanned the subproteome of the outer
membrane and discovered around one hundred proteins,” said Meisinger. “It is interesting to
note that more than ten per cent of these proteins are signalling molecules.” For a long time
scientists had assumed that mitochondria simply supply the energy that is required by the cells
and otherwise have little to do with the rest of the cell. But Meisinger’s findings have shown
that the mitochondria are an essential signalling platform in the cells’ interior.
Communication and the inevitable suicide
Other investigations showed that a large percentage of mitochondrial proteins possess
phosphorylation sites, areas to which small phosphate groups can be attached.
Phosphorylation is typical for molecules that are involved in signalling cascades. “This shows
us that there are many signal transduction pathways in mitochondria,” said Meisinger. And
these signalling pathways are connected with the rest of the cell via proteins in the outer
mitochondrial membrane. The example of signalling molecules that trigger programmed cell
death clearly shows this. For example, signalling molecules transmit information from the cell
nucleus about severe DNA damage caused upon exposure to excessive UV light. They activate
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membrane proteins in the outer mitochondrial membrane that transfer the suicide signal into
the interior of mitochondria, which react to the signal by releasing cytochrome C into the
cytosol. This is the moment when the cell starts to undergo programmed cell death and there
is no way back.
“We are currently investigating the proteins in the outer mitochondrial membrane with which
the suicide signals interact and what subsequently happens inside the mitochondria on the
molecular level,” said Meisinger. The researchers found that SAM and TOM also interact with
signalling pathways and will focus on these interactions in future projects. Something that has
already become clear is that the mitochondria have long been underestimated and that they
might turn out to be of great importance for the pharmaceutical industry. And since the
proteins in the outer mitochondrial membrane are key for the function of mitochondria, they
will most likely also increasingly come into the focus of medical and pharmaceutical research.
Further information:
Prof. Dr. Chris Meisinger
Institute of Biochemistry and Molecular Biology, and
Excellence Cluster for Biological Signalling Studies (bioss)
Stefan-Meier-Straße 17
D-79104 Freiburg
Tel.: +49 (0)761/203-5287
E-mail: chris.meisinger(at)biochemie.uni-freiburg.de
Article
10-Jul-2010
mn
BioRegion Freiburg
© BIOPRO Baden-Württemberg GmbH
The article is part of the following dossiers
Membrane proteins
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