Figure 2.

Formation of functional respiratory complexes in the presence (A) and absence (B) of mtDNA. (A) In the presence of adequate mtDNA transcription and translation, the nuclear- encoded subunits enter the mitochondria through the outer (TOM) and inner (TIM) mitochondrial membrane transporters and combine stoichiometrically with mitochondrially-encoded subunits to form functional respiratory complexes. (B) In the absence of mtDNA, no mitochondrially-encoded subunits are synthesized, and nuclear-encoded subunits are either directly degraded by the cytosolic UPR through the proteasome or enter mitochondria and are degraded by the mitochondrial UPR via mitochondrial proteases. Some of these subunits leave the mitochondria again through reverse transport through TOM and TIM and are degraded by the cytosolic proteasome.