Date: 18.02.2020

Anaerobic peroxisomes in Mastigamoeba balamuthi

Significance
It is generally accepted that peroxisomes are absent in anaerobic eukaryotes. These organelles have evolved to compartmentalize oxidative pathways and prevent cellular oxidative damage, namely, the β-oxidation of fatty acids that utilizes molecular oxygen and produces hydrogen peroxide. Mitochondria possess a parallel β-oxidation pathway coupled with the respiratory chain. The adaptation of eukaryotes to anaerobiosis is reflected by the reduction of mitochondrial metabolism and, not surprisingly, concomitant loss of peroxisomes. The free-living anaerobic protist Mastigamoeba balamuthi is an organism that contradicts this paradigm. Although Mastigamoeba possesses hydrogenosomes, an anaerobic form of mitochondria, it also harbors peroxisomes. These organelles contain the archetypical Pexs that are required for peroxisomal biogenesis; however, they lack the hallmarks of peroxisomal metabolism, β-oxidation and catalase.

Abstract
The adaptation of eukaryotic cells to anaerobic conditions is reflected by substantial changes to mitochondrial metabolism and functional reduction. Hydrogenosomes belong among the most modified mitochondrial derivative and generate molecular hydrogen concomitant with ATP synthesis. The reduction of mitochondria is frequently associated with loss of peroxisomes, which compartmentalize pathways that generate reactive oxygen species (ROS) and thus protect against cellular damage. The biogenesis and function of peroxisomes are tightly coupled with mitochondria. These organelles share fission machinery components, oxidative metabolism pathways, ROS scavenging activities, and some metabolites. The loss of peroxisomes in eukaryotes with reduced mitochondria is thus not unexpected. Surprisingly, we identified peroxisomes in the anaerobic, hydrogenosome-bearing protist Mastigamoeba balamuthi. We found a conserved set of peroxin (Pex) proteins that are required for protein import, peroxisomal growth, and division. Key membrane-associated Pexs (MbPex3, MbPex11, and MbPex14) were visualized in numerous vesicles distinct from hydrogenosomes, the endoplasmic reticulum (ER), and Golgi complex. Proteomic analysis of cellular fractions and prediction of peroxisomal targeting signals (PTS1/PTS2) identified 51 putative peroxisomal matrix proteins. Expression of selected proteins in Saccharomyces cerevisiae revealed specific targeting to peroxisomes. The matrix proteins identified included components of acyl-CoA and carbohydrate metabolism and pyrimidine and CoA biosynthesis, whereas no components related to either β-oxidation or catalase were present. In conclusion, we identified a subclass of peroxisomes, named “anaerobic” peroxisomes that shift the current paradigm and turn attention to the reductive evolution of peroxisomes in anaerobic organisms.

Le T., Žárský V., Nývltová E., Rada P., Harant K., Vancová M., Verner Z., Hrdý I., Tachezy J. 2020: Anaerobic peroxisomes in Mastigamoeba balamuthi. PNAS 117: 2065-2075. [IF= 9.580]

DOI: 10.1073/pnas.1909755117

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