Metamonada is a eukaryotic supergroup of free-living and parasitic anaerobic protists. Their characteristic feature is the presence of highly reduced mitochondria that have lost the ability to produce ATP by oxidative phosphorylation and in some cases even by substrate phosphorylation, with all ATP being imported from the cytosol. Given this striking difference in cellular ATP metabolism when compared to aerobic mitochondria, we studied the presence of mitochondrial carrier proteins (MCPs) mediating the transport of ATP across the inner mitochondrial membrane. Our bioinformatic analyses revealed remarkable reduction of MCP repertoire in Metamonada with striking loss of the major ADP/ATP carrier (AAC). Instead, nearly all species retained carriers orthologous to human SLC25A43 protein, a little-characterized MCP. Heterologous expression of metamonad SLC25A43 carriers confirmed their mitochondrial localization, and functional analysis revealed that SLC25A43 orthologues represent a distinct group of ATP transporters, which we designate as ATP-importing carriers (AIC). Together, our findings suggest that AIC facilitate the ATP import into highly reduced anaerobic mitochondria, compensating for their diminished or absent energy metabolism.

• Keywords
• ADP/ATP carrier, Metamonada, SLC25A43, mitochondrial carrier protein, mitochondrial evolution, mitochondrion-related organelle,
• MeSH
• Adenosine Triphosphate * metabolism   MeSH
• Anaerobiosis MeSH
• Phylogeny MeSH
• Humans MeSH
• Mitochondrial ADP, ATP Translocases * metabolism   genetics   MeSH
• Mitochondrial Proteins * metabolism   genetics   MeSH
• Mitochondria * metabolism   MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Adenosine Triphosphate * MeSH
• Mitochondrial ADP, ATP Translocases * MeSH
• Mitochondrial Proteins * MeSH

Myeloid leukemia factor 1 (Mlf1) was identified as a proto-oncoprotein that affects hematopoietic differentiation in humans. However, its cellular function remains elusive, spanning roles from cell cycle regulation to modulation of protein aggregate formation and participation in ciliogenesis. Given that structurally conserved homologs of Mlf1 can be found across the eukaryotic tree of life, we decided to characterize its cellular role underlying this phenotypic pleiotropy. Using a model of the unicellular eukaryote Giardia intestinalis, we demonstrate that its Mlf1 homolog (GiMlf) mainly localizes to two types of cytosolic foci: microtubular structures, where it interacts with Hsp40, and ubiquitin-rich, membraneless compartments, found adjacent to mitochondrion-related organelles known as mitosomes, containing the 26S proteasome regulatory subunit 4. Upon cellular stress, GiMlf either relocates to the affected compartment or disperses across the cytoplasm, subsequently accumulating into enlarged foci during the recovery phase. In vitro assays suggest that GiMlf can be recruited to membranes through its affinity for signaling phospholipids. Importantly, cytosolic foci diminish in the gimlf knockout strain, which exhibits extensive proteomic changes indicative of compromised proteostasis. Consistent with data from other cellular systems, we propose that Mlf acts in the response to proteotoxic stress by mediating the formation of function-specific foci for protein folding and degradation.

• MeSH
• Giardia lamblia * metabolism   MeSH
• Humans MeSH
• Proteolysis * MeSH
• Protozoan Proteins * metabolism   genetics   MeSH
• Protein Folding * MeSH
• Check Tag
• Humans MeSH
• Publication type
• Journal Article MeSH
• Names of Substances
• Protozoan Proteins * MeSH