The transcription factor ANAC017 links mitochondrial retrograde signaling with the ubiquitin-proteasome system to control mitochondrial function in Arabidopsis

Mitochondrial biogenesis requires the import of more than a thousand proteins encoded by nuclear DNA. The translocase of the outer mitochondrial membrane (TOM) complex serves as the primary gateway for specific recognition of precursor proteins, which are synthesized in the cytosol. Little is known about the regulation of the abundance of the TOM complex. Using forward genetics, we identified key 26S proteasome subunits, including REGULATORY PARTICLE NON-ATPASE1A (RPN1A), that affect the abundance of TOM-complex subunits through the ubiquitin-proteasome pathway. Loss of proteasome function through rpn1a mutation or MG132 treatment increased the abundance of TOM20 isoforms and induced mitochondrial stress marker genes. By contrast, overexpression of ANAC017, an endoplasmic reticulum-anchored transcription factor that activates mitochondrial retrograde signaling under stress, lowered TOM20 abundance and reduced mitochondrial protein import. The rates of mitochondrial protein import and respiratory activity were also altered. Genetic analyses placed the proteasome downstream of ANAC017, since the reduction in TOM20 required the RPN1a subunit. Transcriptome profiling after antimycin A treatment showed broad ANAC017-dependent reprogramming of ubiquitin-proteasome system genes. A second tier formed by ANAC053-and ANAC078-bound promoters of proteasome subunits, including RPN1a, is required to restrain TOM20 accumulation. These findings establish a two-step transcriptional circuit that engages the ubiquitin-protea some system to tune TOM abundance and coordinate protein import with organelle function.