Reduced mitochondria provide an essential function for the cytosolic methionine cycle

It has been long hypothesised that mitochondrial reduction is intrinsically related to the remodelling of Fe-S clusters assembly. Yet as our knowledge of divergent free-living protists broadens, so does the spectrum of variability within the range of mitochondrial-related organelles (MROs) fundamental functions. We resolved to high precision the MRO proteome of Paratrimastix pyriformis using Localisation of Organelle Proteins by Isotope Tagging (LOPIT) and demonstrate its role in the synthesis of folate derivates bearing one-carbon (1C) units, its link to the glycine cleavage system (GCS) and their only conceivable role as suppliers for the cytosolic methionine cycle, involved in recycling of S-adenosine methionine. This observation provides congruity to the presence of GCS in MROs of free-living anaerobes and its absence in endobionts, which typically lose the methionine cycle and, in the case of oxymonads, also mitochondria. ### Competing Interest Statement The authors have declared no competing interest.