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  • Triple Helix

Insertases: Gateways To the Mitochondria

Written by Nicolas Kim '26

Edited by Surya Khatri '24

Image Source: Stock Images


Introduction

“The mitochondria is the powerhouse of the cell!” is the well-known expression drearily chanted across countless high school biology classrooms, bringing to mind everything from colorful diagrams of the Krebs cycle and ATP synthase to nightmarishly convoluted energetic pathways. However, in spite of its widespread accessibility, this phrase - while accurate enough - nevertheless fails at describing the myriad other regulatory functions that mitochondria provide for the cell. A more accurate description of the mitochondria’s functionality therefore requires a retracing of its evolutionary origins – seeing the formation, qualification, and overall change in its various characteristics over time in accordance with the demands of the cell [1].


Mitochondria: Evolutionary Origins

Rewinding the evolutionary clock, the mitochondria of today started off as bacteria living symbiotically within eukaryotic cells. Over the course of their evolution, their form and function drastically changed along with their genetic material, leading to their retention of certain characteristics – such as their original role in ATP production and double membrane structure – but the development of many more functions, ranging from the regulation of cell death (apoptosis) to other multi-functional signaling pathways terminating in distant parts of the cell. To regulate these ever-changing functions, the mitochondria evolved to synthesize a greater diversity of regulatory proteins - ranging from the 13 proteins used to modulate ATP synthesis to thousands more for additional pathways. These proteins are ultimately the keys to mitochondrial structure and function, but the question of how they’re synthesized – much less how they end up where they do in humans – has remained unresolved [1].


In yeast and similar eukaryotic organisms, it has been well-defined that there are certain proteins embedded in the mitochondrial membrane that allow for other regulatory factors (namely those also embedded in the membrane) to get inserted into the membrane and thus function properly. However, the question of whether or not humans have equivalent enzymatic players (known as insertases) to allow for the similar insertion of their regulatory proteins has long remained unanswered. Identifying these players - as well as understanding the biochemical processes by which they insert their substrates - provides a long-awaited explanation for their cellular significance, as well as defining their potential for future treatments exploiting their functionality.


Breakthroughs

Now, a landmark study has revealed that a protein named mitochondrial carrier homolog 2 (or MTCH2) is the likely player necessary for the insertion of various alpha-helical proteins into the mitochondrial membrane [2]. Previous studies found that MTCH2 loss was positively correlated with cancer, Alzheimer’s, and the dysregulation of numerous cellular pathways – but no one knew why, especially as the function of MTCH2 remained enigmatic [3].


In this study, the scientists first used a genetic screen of their own design (called CRISPR-i), wherein they removed the genes that they hypothesized were responsible for synthesizing potential insertase candidates. They then evaluated how certain transmembrane mitochondrial protein complexes, requiring the action of insertases, responded to the removal of said “insertases.” From this screen, they were able to conclude that MTCH2 was the likely insertase they were looking for, given that the transmembrane protein of interest (they were trying to insert) was not integrated into the membrane in the absence of MTCH2.


However, this result alone was not sufficient to explain the various, seemingly unrelated, effects – such as the simultaneously increased risk of Alzheimer’s and cancer – resulting from the loss of MTCH2. To resolve this, the scientists subsequently conducted an assay where they tested the ability of MTCH2 to integrate a family of alpha helical proteins – all regulating different pathways in the cell (with the dysregulation of each pathway leading to the aforementioned

effects) – into the mitochondrial membrane. Upon the depletion of MTCH2, they again found that MTCH2 was necessary and now sufficient for the insertion of these proteins. These results in turn provided explanatory power for the variegated effects of MTCH2 loss in the cell.


Applications

Consequently, the study concluded that MTCH2 was likely their hypothesized insertase. Since the protein assay described above revealed that MTCH2 was responsible for the insertion, and proper activity, of proteins regulating apoptotic pathways, the scientists wondered if an additional application of MTCH2 deletion could be the regulation of cancer pathways (since cell death, or apoptotic, pathways would ideally be inhibited for tumor cells). Consequently, the scientists created two separate experimental cell groups for this application – one for the overexpression of MTCH2 and one for the deletion of MTCH2 – and added anti-cancer drugs to each group. Their results revealed that the deletion of MTCH2 did not have a significant effect in increasing the propensity for apoptosis, but its overexpression made the cancer cells more sensitive to the anti-cancer drug treatment.


Looking Forward: Concluding Remarks

From these results, the study was able to find both a potential, likely, candidate for a mitochondrial insertase in humans, in addition to a viable application of said insertase to anti-cancer treatments. Given the various regulatory roles that MTCH2 has in the cell, the tentative discovery of this protein heralds the possibility of many more therapeutic treatments, as well as the chance to trace back its evolutionary origins more comprehensively [2, 3].

 

References

1. Vögtle, F. N., Koch, H. G., & Meisinger, C. (2022). A common evolutionary origin reveals fundamental principles of protein insertases. PLoS biology, 20(3), e3001558. https://doi.org/10.1371/journal.pbio.3001558

2. Guna, A., Stevens, T. A., Inglis, A. J., Replogle, J. M., Esantsi, T. K., Muthukumar, G., Shaffer, K. C. L., Wang, M. L., Pogson, A. N., Jones, J. J., Lomenick, B., Chou, T. F., Weissman, J. S., & Voorhees, R. M. (2022). MTCH2 is a mitochondrial outer membrane protein insertase. Science (New York, N.Y.), 378(6617), 317–322. https://doi.org/10.1126/science.add1856

3. Eaglesfield, R., & Tokatlidis, K. (2021). Targeting and Insertion of Membrane Proteins in Mitochondria. Frontiers in cell and developmental biology, 9, 803205. https://doi.org/10.3389/fcell.2021.803205

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