Interconnections with and within the trypanosomal respiratory chain revealed by complexome profiling

The study of proteins often reveals their significance through their interactions within complex networks. Proteins, while individually important, derive their function from the company they keep within these complexes. However, proteomes, which are protein inventories, lack information about the three-dimensional structures of these protein assemblies.

To overcome this limitation, complexome profiling (CP) has emerged as a transformative tool, enabling the capture of the comprehensive population structure of protein complexes within a cell at a given moment. Researchers have used CP to map the multiprotein complex landscape of two life cycle stages of Trypanosoma brucei, a protozoan parasite of medical and evolutionary importance.

Through this analysis, the team uncovered previously hidden interactions within and among the components of the mitochondrial respiratory chain, a crucial energy-producing system within the cell. Three key findings emerged from their CP data:

Firstly, they identified an exceptional case of a SLC25 solute transporter that unexpectedly interacts with NADH dehydrogenase, a key enzyme in the respiratory chain. This interaction was not previously recognized.

Secondly, two ATP synthase subunit g paralogs, proteins that play a role in ATP production, were found to be intriguingly excluded from the enzyme's dimers, or protein complexes. The inclusion of these subunits in the complex structure was previously unknown.

Lastly, the researchers refined the known composition of ubiquinol:cytochrome c oxidoreductase, another enzyme in the respiratory chain, by adding missing subunits and removing an incorrectly assigned subunit. This refinement more likely identifies the subunit responsible for inserting the iron-sulfur co-factor into the complex, a critical step in the enzyme's function.

Further investigation into ubiquinol:cytochrome c oxidoreductase assembly revealed an unforeseen crosstalk between the incorporation of its nuclear subunits and mitochondrial translation. This crosstalk may facilitate an unknown quality control mechanism, ensuring the proper assembly and function of the enzyme.

These discoveries underscore the power of CP data in generating and testing hypotheses about the complex networks of mitochondria and other organellar multiprotein complexes within T. brucei. Such insights are crucial for understanding the biology of this medically significant protozoan, as well as for illuminating evolutionary relationships among related species.