Nature publication: Biochemists clear up a decades-old misconception about a key metabolic pathway

Imagine that the brightest minds in a particular discipline all agree that the object of their research looks like a triangle. What then happens when someone turns up and says: 'No, it's actually a square.'? 'They'll say he's cuckoo!' said Martin van der Laan, whose unequivocal response directly reflects the experience that his colleague Eunyong Park from the University of California in Berkeley made about two years ago. 'He published a manuscript in which he depicted the structure of the TIM23-TIM17 complex that was quite different from what nearly every expert in the field had assumed. No biochemical experiment of the previous 25 years seemed to fit with this new structure,' explained van der Laan, professor of medical biochemistry at Saarland University whose main area of research is mitochondria – the 'powerhouses' that drive cellular metabolism. The experts were unanimous: Their research colleague in California must be mistaken.

But it turns out he wasn't. Up until recently, scientific doctrine held that the protein complex TIM23 (TIM stands for 'translocase of the inner membrane') forms a tunnel-like structure through which large protein molecules can be transported into the mitochondria from other parts of the cell – something that van der Laan demonstrated by placing his two cupped hands together with their fingertips touching. This hollow channel through the mitochondrial envelope acts like a keyhole that will only accept a molecule that has the right key. When such a molecule approaches, it gets pulled into the channel and transported, together with energy-supplying auxiliary proteins, into the interior of the mitochondrion. That, at least, was the established scientific paradigm for decades.

To help understand why this model is not correct, Martin van der Laan now placed his cupped hands back to back with his fingers now pointing outwards. It was this new structure, which Eunyong Park had proposed on the basis of high-resolution cryo-electron microscopy data, that seemed so utterly surprising. The structure of the transport complex looked completely different from what had been assumed for decades. In this new structure, one of the hands represents TIM23, while the other is its 'fraternal twin' TIM17. But up until then, the TIM17 protein had not really played any significant part in describing the protein transport mechanism in mitochondria. It was thought that TIM17 had more of a supporting or regulatory role in the transport of proteins through the mitochondrial membrane, and that the real star of the team was TIM23.

It turns out that's not the case. Martin van der Laan and his colleague Nils Wiedemann from Freiburg have been collaborating closely for almost two decades. Their research teams recently took another close look at the TIM17-TIM23 complex and have now mapped the functional organization of the complex in great detail using advanced and highly precise biochemical methods. A key feature of this work was the re-evaluation of old data that previously had made little sense, but that now fitted perfectly with this revolutionary new picture of the TIM complex.

The results of the research work carried out in Homburg and Freiburg have completely upturned the previous long-held assumptions about how proteins get inside mitochondria, reinforcing the conclusions drawn from the structural investigations by the research group at UC Berkeley. Martin van der Laan summarized these new findings: 'Taking the new structural and biochemical data together, we can now see that proteins migrate into the mitochondria along a pan-shaped membrane opening formed by TIM17 and not via a TIM23 channel structure.' The supporting actor has suddenly become the star of the show.

According to van der Laan, it was an ingenious experimental trick that has been crucial to this biochemical paradigm shift. 'We made an artificial mitochondrial protein that gets stuck in the TIM17/23 transport pore like a cork getting stuck in the neck of a wine bottle. We then modified the trapped protein complex so that free radicals – highly reactive chemical groups – were released on our artificially engineered protein. The reaction of these free radicals with their molecular surroundings is something that we can observe with extremely high spatial resolution. What we found was that the free radicals were only active in the TIM17 half-channel,' explained Professor van der Laan. This could only mean that the mitochondrial proteins migrate across the envelope membrane in close contact with the TIM17 structure, and not via a TIM23 channel, which is the mechanism presented in practically all the standard textbooks on biochemistry and cell biology.

That alone is a pretty revolutionary discovery. But why is this finding of such importance – and not just to a handful of specialist researchers around the world? As Martin van der Laan puts it: 'Mitochondrial dysfunction can result in severe degenerative and metabolic diseases and is known to be involved in the development of Parkinson's disease, diabetes and certain types of cancer.' Improving our understanding of how proteins actually enter the mitochondria – the 'powerhouses' that maintain cellular energy supply – could facilitate the development of highly effective drugs that are better able to treat those suffering from such serious diseases.

This article is the second paper that Martin van der Laan and his research group have published in Nature, one of the world's top scientific journals (link to first paper)