The TIM complex not irreducibly so

Again, in this loose series of posts based around the fact that evolution delivers the scientific goods, I present you all with more evidence for evolutionary theory – this time revolving not around the teeth of whales, but the biomachines of bacteria.

Most Australian readers are aware that our fine country falls a bit behind in terms of science funding, so it’s always good to know that at least once in a while something awesome is propelled from our universities. In fact, this piece of research comes out of Monash University, my second preference for tertiary study next year. Small world. Kinda.

Professor Trevor Lithgow, along with a team of researchers from Monash and a few other universities, have provided the scientific community with yet more evidence for evolution’s validity as a theory to explain the diversity of life on Earth. Proving that the devil really is in the details, they dived into the world of mitochondria, the organelles that provide eukaryotic cells with life-sustaining ATP – the breakfast (read: energy source) of champions (read: all life’s biochemical pathways).

For those not hip with mitochondria, the prevailing theory for the origin of these complicated organelles is that they were once, deep in evolutionary history, self-contained bacteria, existing apart from the ancestors of modern eukaryotes. Endosymbiotic theory, as it is known, has lots of supporting evidence, ranging from the fact that mitochondria and chloroplasts, which are also considered endosymbionts, reproduce by binary fission during mitosis, like bacteria do; to the presence of separate genomes within the organelles that have far more in common with bacteria than eukaryotes.

So, what do mitochondria have to offer the realm of evidence for evolution? Let me introduce you to the Transporter Inner Membrane complex, or TIM complex for short. This biomachine, for the lack of a better, non-jargony word, transports proteins across the inner membrane of mitochondria. There is, of course, a TOM (Transporter Outer Membrane) complex, but this research didn’t look at that – something that could be easily fixed with a little more science funding…

Now, TIM complexes don’t exist in the modern day non-endosymbiotic ancestors of mitochondria. So, how did they come about? Evolution is great process, but it rarely creates things from absolutely nothing. Often in bacterial evolution, proteins are co-opted for various different roles in the cell, usually only after slight mutational modification. The bacterial flagellum is a great example of this – proteins that make up part of this movement-oriented structure are also part of the Type III secretion system. Could the TIM complex have come about through the modification of other bacterial proteins?

According to Professor Lithgow, yes! By studying the bacterium Caulobacter crescentus, a species that has similar characteristics to the ancestral pre-symbiotic mitochondrion, they discovered proteins that are extremely similar to the TIM complex, yet are not involved with intra- or inter-cellular protein transport mechanisms. In more complicated terms, proteins similar to the TIM44 subunit is found in all alpha-proteobacteria, as are ones structurally close to the TIM14 subunit. Genetic experimentation was also undertaken to show that only minimal mutational differences are needed to allow the bacterial equivalent of TIM14 to function within the TIM23 complex.

This provides excellent evidence that the proteins that make up the TIM complex evolved from other proteins within the bacterial cell. Evolutionary theory predicts that such similarities should be found in the inner workings of every cell, so to find such evidence points strongly towards evolution taking place.

Such evidence really puts a fatal puncture in the common intelligent design proponent claim that certain structures, if not all structures, within cells cannot be reduced down to a lower level of evolutionary complexity – if the TIM complex and the flagellum were evolved through the modification of already existing structures, then there is no problem with their evolutionary history.

All good research generates predictions, so, because of the puncture-inducing nature of this study, I predict that there will be some mention of this research on one of the main pro-ID blogs – Uncommon Descent, Darwin’s God, Evolution News & Views, etc. – slamming it and saying: “Arguments from irreducibly complex systems still hold water – I mean, seriously, where did those precursor proteins come from?”

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Yeah, right. I’ll be sure to keep you informed on any action in the enemy’s camp. You can count on me with the binoculars.

Very special thanks to @Jen_Bennett on Twitter for graciously sending me the full version of this academic paper. Follow her, everyone!