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  • Alex Cloherty

Evolution, or, the feet in the powerhouse of the cell

Updated: Dec 18, 2021

If you remember anything from high school biology, probably what you remember is that mitochondria are the powerhouse of the cell. But, did you know that mitochondria can give us clues about the very nature, and beginnings, of life?

Well, a new study (first uploaded on bioRxiv and later published here) about the mitochondria of an algal microbe (Chlamydomonas reinhardtii, to be specific) slots in a new piece into the puzzle of evolution, by looking, indeed, inside the mitochondria. But... Why should you care about mitochondria anyways?

You should care, because mitochondria are an important part of what made us humans, us. Without mitochondria, life as we know it could never have gotten so BIG. Mitochondria are thought to be majorly responsible for allowing life to expand from being only single cell organisms, like bacteria, to multicellular organisms (i.e. lots of cells working together to make one living thing) like us. You kind of have mitochondria to thank for the fact that you're not just an amoeba - because they provide the power needed for BIG life.

In the very beginning, before multicellular life, cells got their power in a totally different way. You can think of it kind of as an agricultural age for cellular power. Cells didn't have any centralized powerhouses back then - rather they had a spread out, less efficient system that was fine for their current needs, but didn't allow them too much extra power for non-essential activities. That all changed when one of those "pastoral" cells ate up another cell. The second, gobbled-up cell, became the first mitochondrion.

That's right, mitochondria are basically the remnants of a cell that learned to live inside another cell. And not just to live - to thrive. Upon being gobbled, that gobbled cell could specialize, and it specialized in power. When those two ancient cells learned to work together, they could create enough power to light up a metaphorical city - i.e. to drive an explosion of evolution that eventually led to us.

Interestingly, because mitochondria evolved from a free-living bacterial ancestor (the gobbled cell), they still have some features of bacteria. But, thanks to that evolutionary explosion after the gobbling event (the "endosymbiotic event" in scientific parley), not all mitochondria have the same features. Just like the many different plants, animals, algae, and even amoebas that exist today, the mitochondria within those organisms look unique. And, understanding the differences between those mitochondria can give scientists clues about the story of our evolution - and the evolution of all those different plants, animals, algae, and amoebas alongside us. And, this new study on algae mitochondria writes in a new chapter into that story.

In particular, this study presents two super interesting and novel findings that focus in especially on the ribosomes - which are some of the most important workers that help power the powerhouse - inside the Chlamydomonas reinhardtii mitochondria.

Firstly, the authors of this study show that the structure of these "mitoribosomes" (ribosomes inside mitochondria) are completely unique to any other mitoribosomes described by scientists yet. They have a very cute structure, actually. Basically, the instructions for making the Chlamydomonas reinhardtii mitoribosomes are totally weird. It's as if its recipe for mitoribosomes has come straight out of a Dan Brown novel - it's been all scrambled and fragmented and spread out across this algae's genetic material. But, beautifully, all those pieces are able to come together and sort of hug each other to make functional mitoribosomes again. This is completely different from our human mitoribosome "recipes", which are much more straight forward - rather out of a Jamie Oliver cookbook than a Dan Brown novel.

Secondly, these scientists were able to actually take super high-resolution pictures of these tiny mitoribosomes, using some of the most powerful microscopes we have today (electron microscopes, and they used in situ cryo-electron tomography, for the real nerds out there). This is the first time ever we have been able to see a picture of a mitoribosome from green algae, and the picture is pretty impressive. What's more, it also shows that the Chlamydomonas reinhardtii mitoribosomes are totally different from other known mitoribosomes in a second way! So, this will sound strange, but you'll have to trust me on this: mitoribosomes have feet. Basically, the feet of mitoribosomes can help keep them stuck onto the mitochondria, so they don't go floating off their jobsite. Mitoribosome feet keep mitoribosomes anchored in place, doing their powerhouse jobs. And it turns out, different types of mitoribosomes have different types of feet - and the way that those feet look can give us hints about how the mitoribosomes evolved over the history of life. I kind of like to think of this in the same way as how our macroscopic feet, or other appendages, can give us some clues into evolution. Like, if you look at a human foot, a chimpanzee foot, and a zebra foot, you'd probably guess that we're more closely related to the Chimps than the Zebras. Well, same goes for the mitoribosomes. And interestingly, the mitoribosome feet of this algae also look totally new. Before this new study, we already knew that human mitoribosomes had only one foot (made out of protein), and yeast mitoribosomes had two feet (one foot made of protein, and one made out of RNA - kind of like having one human foot and one zebra hoof). Well, these algae mitoribosomes have two feet like in yeasts, but they are both protein, like the human mitoribosome foot. And why is this important for us? Well, it raises the question, did we humans lose a mitoribosomal foot at some point, and now they're all just hopping around on pogo sticks? Or are our one-footed wonders the original model, and did yeast and algae mitoribosomes grow extra feet at some point? We still don't know - but after all, the best research answers some old questions, but raises some new ones.

Altogether, the research in this new study gives us some beautiful pictures of some very small molecules, and more importantly, gives us new information about the evolution of our powerhouses and the workers within them. And to take a human-centric view, it gives us new information, and new questions, about where our powerhouses stand compared to our far-distant cousins throughout evolution.

To tie up, time for a quick note - this study I'm writing about today was co-authored by my boyfriend, Robert Englmeier. So I have a sort of conflict of interest in making it sound cool to everybody. But nonetheless, sometimes it can be difficult for the non-hard-core-scientist public to see right away why they should be interested in "the fragmented mitoribosomes of Chlamydomonas reinhardtii". So, what do you think by now? Have I convinced you that you should care about tiny machines in mitochondria in algae? Let me know in the comments!

Until next week, take joy in the small stuff.

~ Alex

P.S. An exciting update from December 2021: This study has now been published in Nature Communications! You can check out the peer reviewed and published version here.

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