TOR is an important kinase that integrates information about hormones, nutrients and metabolites. In mammals and yeast two TOR complexes exist, TORC1 and TORC2, of which only the former is conserved in plants. Arabidopsis TORC1 has three core subunits: TOR, RAPTOR and LST8. Its localization has been assayed by transient overexpression and was found to be associated with nuclei, endosomes and the cytosolic compartment. However, much less is known about in vivo targets of this complex and thus about the response downstream of signal perception. In mammalian cells, the homologous mTOR complex was reported to associate with mitochondria and to regulate mitochondrial physiology, and actin dynamics were recently reported to correlate with TOR activity.
The researchers show that Arabidopsis TORC1 interacts with the surface of mitochondria and regulates ATP levels by affecting the mitochondrial membrane potential. Both the mutant of the core component RAPTOR and the chemical inhibition of TOR led to a reduction in the mitochondrial membrane potential, as well as reduced cytosolic ATP levels. It was furthermore shown that the reduced ATP level was the direct cause of reduced actin dynamics. Interestingly, the organization of the actin network in TORC1-impaired plants resembled the wild-type structure, suggesting that low ATP preserves the cytoskeleton in an overall favourable but less-dynamic state. It can be speculated that this reduced actin dynamic might not just be a passive consequence of reduced ATP levels but an actively regulated strategy to save ATP for other essential processes such as the maintenance of cellular homeostasis.
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