ATG8 homologs, TtATG8A and TtATG8B, are responsible for mitochondrial degradation induced by starvation.

UNLABELLED

The majority of heterotrophic unicellular eukaryotes have evolved mechanisms to survive periods of starvation, allowing them to endure until conditions are favorable for regrowth. The ciliate exhibits active swimming behavior in water, preying on microorganisms and growing exponentially at a rate of 0.5-0.75 h⁻¹ under optimal conditions. In this organism, numerous mitochondria localize to the cell cortex along the ciliary rows, likely ensuring an efficient ATP supply necessary for vigorous cell movement. Although mitochondrial reduction occurs immediately under starvation, the underlying mechanism remains unknown. Here, we demonstrated that autophagy is responsible for mitochondrial reduction in . Among the five ATG8 homologs, TtATG8A and TtATG8B formed granule- and cup-shaped structures in response to starvation. Fluorescent microscopy further showed that TtATG8A and TtATG8B associate with mitochondria. Moreover, correlative light and electron microscopy analysis revealed that mitochondria colocalized with TtATG8A or TtATG8B were engulfed by autophagosomes and displayed abnormal appearances with disrupted cristae structures. Additionally, repression of TtATG8A or TtATG8B expression significantly attenuated starvation-induced mitochondrial reduction. These findings suggest that TtATG8A- and TtATG8B-mediated autophagy is a key mechanism underlying mitochondrial reduction in starved .

IMPORTANCE

This study is the first comprehensive description of the mitochondrial degradation process under nutrient starvation in the ciliate . It is well known that the cell surface structure of ciliates consists of an elaborate spatial arrangement of microtubule networks and associated structures and that this surface repetitive pattern is inherited by the next generation of cells like genetic information. Our findings provide a basis for understanding how ciliates maintain an adequate amount of mitochondria on the cell surface in response to nutritional conditions. Furthermore, we have successfully demonstrated the usefulness of as an experimental system for studying mitochondrial quality control and turnover. Further studies of will facilitate comparative studies among diverse biological systems on how eukaryotes other than opisthokonta (yeast, cultured cells, etc.) control their mitochondria.