Category Archives: organelles

More ‘Evidence’ of Front-loading Mitochondria

I have previously shared some evidence for the front-loading of mitochondria, along with an emerging picture where symbiogenesis is merged with front-loading. Now there is yet some more evidence in support of the plausibility of front-loading mitochondria.  Those who follow this blog, or have read The Design Matrix, will note how seamlessly this all fits in with my hypothesis of life’s design:

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How to Guide the Blind Watchmaker

In the past, I have provided multiple lines of evidence to establish the plausibility of front-loading evolution.However, we have focused primarily on the transition from a unicellular to a multicellular eukaryotic state.Let’s now take one step back and begin pondering whether eukaryotic cells were themselves front-loaded to appear. For without the eukaryotic cell design, it is unlikely that the planet would possess anything analogous to an animal or plant.

While we still don’t really understand the origin of the eukaryotic cell, there is a strong consensus about the origin of the mitochondria, a very important organelle of eukaryotes.According to the endosymbiotic theory, mitochondria are the descendents of bacteria. The theory postulates that a phagocytic cell engulfed some aerobic bacteria and rather than digest them, a symbiotic relationship was established, where each partner benefited from the new relationship. This relationship then set the stage for the ultimate stream-lining of the bacteria, such that they were transformed into mitochondria through the transfer of much of their gadgetry to the host nucleus.

In a nutshell, the essence of the argument for the endosymbiotic origin of mitochondria is that mitochondria look like they share a common ancestor with bacteria. The argument is quite convincing, as there are numerous mitochondrial genes whose sequences are much more similar to bacterial sequence than that which exists in the nucleus of the same cell. In fact, this is an example where no one piece of evidence carries the day, but instead it’s the cumulative power of multiple lines of evidence.

Since mitochondria were once bacteria, might this transition have been front-loaded to happen?

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The Tale of Tom20

Let’s take a look at Tom20, the protein that acts as a receptor to snag the mitochondrial targeting sequence (MTS) and shuttle it to the membrane channel.

Tom20 has been studied in fungal and mammalian systems and shown to have the a domain structure as shown in Figure 1.

tom20a

Figure 1. Domain organization of Tom20 in animals and fungi (modified from Fig 5 of reference listed below). N = amino terminus (the front-end of the protein) and C = carboxyl terminus (the back end of the protein).

The gray boxes represent regions that are disordered, the blue box represents a TPR domain that binds the MTS, and the black box represents the region that spans the outer membrane of the mitochondria. The line above represents the regions exposed to the cytoplasm.

So the black box region sits in the membrane and the blue box, with TPR domains, snags the MTS.

When scientists used the sequence of this protein to query databases for homologs in plants and protozoa, nothing was found. Could it be that plants and fungi don’t have Tom20?

No.

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Meet Tom

We have just seen that bacterial cells were preadapted for the formation of mitochondria. More specifically, one out of every twenty bacterial proteins harbors the equivalent of the mitochondrial targeting sequence which is needed for entry into the mitochondria. But this story gets more interesting. To appreciate the twist that comes next, let’s step back to make sure we can visualize the process of transporting mitochondrial proteins into the mitochondria.

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Front-loading mitochondria

In the past, I have provided multiple lines of evidence to establish the plausibility of front-loading evolution. However, we have focused primarily on the transition from a unicellular to a multicellular eukaryotic state. Let’s now take one step back and begin pondering whether eukaryotic cells were themselves front-loaded to appear. For without the eukaryotic cell design, it is unlikely that the planet would possess anything analogous to an animal or plant.

While we still don’t really understand the origin of the eukaryotic cell, there is a strong consensus about the origin of the mitochondria, a very important organelle of eukaryotes. According to the endosymbiotic theory, mitochondria are the descendents of bacteria. The theory postulates that a phagocytic cell engulfed some aerobic bacteria and rather than digest them, a symbiotic relationship was established, where each partner benefited from the new relationship. This relationship then set the stage for the ultimate stream-lining of the bacteria, such that they were transformed into mitochondria through the transfer of much of their gadgetry to the host nucleus.

In a nutshell, the essence of the argument for the endosymbiotic origin of mitochondria is that mitochondria look like they share a common ancestor with bacteria. The argument is quite convincing, as there are numerous mitochondrial genes whose sequences are much more similar to bacterial sequence than that which exists in the nucleus of the same cell. In fact, this is an example where no one piece of evidence carries the day, but instead it’s the cumulative power of multiple lines of evidence.

Since mitochondria were once bacteria, might this transition have been front-loaded to happen?

Continue reading