Mitochondria-giant lipid droplet proximity and autophagy suppression in nitrogen-depleted oleaginous yeast Lipomyces starkeyi cells.

Balancing energy production and storage is a fundamental process critical for cellular homeostasis in most eukaryotes that relies on the intimate interplay between mitochondria and lipid droplets. In the oleaginous yeast Lipomyces starkeyi under nitrogen starvation, lipid droplet forms a single giant spherical structure that is easily visible under a light microscope. Currently, how mitochondria behave in L. starkeyi cells undergoing giant lipid droplet formation remains unknown. Here we show that mitochondria transition from fragments to elongated tubules and sheet-like structures that are in close proximity to a giant lipid droplet in nitrogen-depleted L. starkeyi cells. Under the same conditions, mitochondrial degradation and autophagy are strongly suppressed, suggesting that these catabolic events are not required for giant lipid droplet formation. Conversely, carbon-depleted cells suppress mitochondrial elongation and lipid droplet expansion, whereas they promote mitochondrial degradation and autophagy. We propose a potential link of mitochondrial proximity and autophagic suppression to giant lipid droplet formation.