Mitochondria's Lost & Found: How Protein Transport Machines Evolved
"Tracing the evolutionary journey of mitochondrial protein transport, revealing surprising losses and adaptations in bacteria-inspired machinery."
Mitochondria, those tiny power plants within our cells, have a fascinating history. Once free-living bacteria, they were incorporated into eukaryotic cells in a process called endosymbiosis. This transformation required a complete overhaul of their protein management system, including how proteins are transported across their membranes.
Bacteria rely on translocases, specialized machines that shuttle proteins across or insert them into their inner and outer membranes. Mitochondria have retained some of these bacterial-derived translocases but have discarded others during evolution. The inner membrane TAT (twin-arginine translocation) translocase, responsible for transporting folded proteins, has seen a sporadic retention across the eukaryote tree.
New research sheds light on the evolutionary path of mitochondrial TAT translocases, revealing how these protein transport machines have adapted—and sometimes disappeared—in mitochondria.
Mitochondrial Protein Import
Proteins encoded in the nucleus are imported into mitochondria by specialized transport machineries in the outer and inner mitochondrial membranes. Their coordinated transport, followed by folding, assembly, or integration into mitochondrial membranes, is central to mitochondrial function. The import system uses diverse molecular mechanisms and is integrated with wider mitochondrial and cellular networks.
Multiple Import Pathways
Mitochondrial biogenesis requires importing approximately 1,000 different precursor proteins into and across mitochondrial membranes. The transport and sorting machinery includes five major import pathways that deliver proteins to their functional destinations within the organelle. This scale and diversity mean there is no single mechanism for all precursor proteins; the review describes a wide variety of pathways and machineries.
The Ups and Downs of TAT Translocases: An Evolutionary Tale
The study, published in BMC Biology, delves into the distribution and function of mitochondrial TAT subunits across various eukaryotic lineages. Researchers identified three distinct types of TatABC-derived machineries encoded by eukaryotes: TatAC, TatBC, and TatC-only. The focus here is on TatAC and TatC-only machineries, which haven't been extensively studied before.
- Mitochondria inherited three inner membrane translocases: Sec, TAT, and Oxa1 (YidC) from their bacterial ancestor.
- Mitochondrial TAT has likely retained its unique function of transporting folded proteins, at least in some eukaryotes.
- Mitochondria have abandoned the TAT machinery multiple times during evolution, unlike chloroplasts.
- Oxa1 translocase was nearly universally retained in mitochondrial biogenesis pathways.
The Uneven Distribution of Tat
The twin-arginine translocation (Tat) machinery is found in prokaryotic membranes and is evolutionarily conserved in chloroplast thylakoid membranes and the inner membrane of some mitochondria. In plants, Tat machinery is conserved in chloroplasts and mitochondria, while animal mitochondria have generally lost it, with homoscleromorph sponges noted as an exception. More broadly, many mitochondria lost Tat during evolution from their bacterial ancestor. Reports also identify mitochondrial Tat components in heteroloboseans and the alveid *A. twista*.
Unresolved Function and Import Vulnerability
A study of mitochondrial Tat translocases reports that one gene is slightly longer than its prokaryotic counterparts, while the role of the whole mitochondrial complex remains unknown. More generally, import is a multistep process vulnerable to defects in cytosolic quality control, the import machinery, and organelle function. The ubiquitin-proteasome system in the cytosol contributes to surveillance of import via the TOM complex, working in coordination with mitochondria-localized and cytosolic proteins to prevent proteotoxic stress.
Independent Evolution and Gene Loss
Mitochondrial evolution proceeded independently in different eukaryotic lineages, producing diversity in mitochondrial genomes and in the mechanisms by which their genes are expressed. Mitochondria have lost most genes inherited from their bacterial ancestor, either through transfer to the nucleus or elimination. Despite this extensive loss, mitochondria in almost all eukaryotic cells retain mitochondrial genomes and gene-expression mechanisms.
What Does This Mean for Understanding Cellular Evolution?
This research highlights the dynamic nature of mitochondrial evolution. At their origin, mitochondria inherited three key protein transport systems from their bacterial ancestors. However, over eons, some of these systems were lost or adapted, revealing the selective pressures that shaped these organelles.
Mapping Mitochondrial Diversity
A research consortium has generated what its report describes as the broadest look yet at the proteins that make up mitochondria. The project captures mitochondrial diversity across multiple branches on the tree of complex life. This broad comparative view provides a wider context for studying how mitochondrial components vary across lineages.
The sporadic retention of the TAT translocase, particularly its unique ability to transport folded proteins, emphasizes its specialized function in certain eukaryotes. The abandonment of this machinery in many mitochondria, contrasted with its successful transfer in chloroplasts, underscores the diverse evolutionary paths taken by these organelles.
Understanding the evolution of these protein transport pathways provides crucial insights into the intricate processes that govern cellular function and the remarkable adaptability of life.