Mitochondrial Protein Abundance Gradients Require the Distribution of Separated Mitochondria

SIMPLE SUMMARY: Individual mitochondria within a cell can be heterogeneous regarding their physiological and morphological characteristics. Gradients in the abundance of proteins from the perinuclear to the peripheral mitochondria have previously been demonstrated. The molecular mechanisms underlying these gradients are unknown. In this study, we demonstrate, through the example of the protein Tom20, a subunit of the translocase of the mitochondrial outer membrane, that abundance gradients are formed quickly after cell division. Moreover, these gradients require separated mitochondria and intact microtubules. This suggests an active process that positions mitochondria according to their properties within the cell. ABSTRACT: Mitochondria are highly dynamic organelles that interchange their contents mediated by fission and fusion. However, it has previously been shown that the mitochondria of cultured human epithelial cells exhibit a gradient in the relative abundance of several proteins, with the perinuclear mitochondria generally exhibiting a higher protein abundance than the peripheral mitochondria. The molecular mechanisms that are required for the establishment and the maintenance of such inner-cellular mitochondrial protein abundance gradients are unknown. We verified the existence of inner-cellular gradients in the abundance of clusters of the mitochondrial outer membrane protein Tom20 in the mitochondria of kidney epithelial cells from an African green monkey (Vero cells) using STED nanoscopy and confocal microscopy. We found that the Tom20 gradients are established immediately after cell division and require the presence of microtubules. Furthermore, the gradients are abrogated in hyperfused mitochondrial networks. Our results suggest that inner-cellular protein abundance gradients from the perinuclear to the peripheral mitochondria are established by the trafficking of individual mitochondria to their respective cellular destination.