Figure 4. The Tom70-dependent regulation of mitochondrial biogenesis is involved in the cellular response to the mitochondrial import defect.

(A) Mitochondrial biogenesis in wild type cells. Metabolic signals regulate the transcriptional activity of mitochondrial proteins, which are synthesized in the cytosol and translocated through the outer mitochondrial membrane (OMM) and inner mitochondrial membrane (IMM) via TOM-TIM complexes. Tom70, a key component of the TOM complex, serves as the import receptor and the biogenesis regulator of mitochondrial proteins. Tim23, a key component of the TIM complex, acts downstream and receives the incoming nascent mitochondrial proteins from the TOM complex. (B) The potential role of Tom70 in the cellular response to the impairment of mitochondrial import caused by tim23^ts. (C) Representative images and quantification for different strains after switching to the restricted temperatures to inactivate tim23^ts. All strains were cultured in raffinose medium at 25 °C overnight, followed by adding galactose for 3hrs before adding glucose for 30 min and switching to 35 °C for 2 hr to inactivate tim23^ts. Cytosolic protein aggregation was visualized with GFP-tagged Hsp104, a general marker of protein aggregates (Glover and Lindquist, 1998; Zhou et al., 2014). Bar graphs are the mean and s.e.m. of the Hsp104-GFP signals inside protein aggregates in each cell.Scale bar: 5 μm. Images are representative of at least two independent experiments. Data were analyzed with unpaired two-tailed t test: ***, p < 0.001; n.s., not significant. Sample sizes in (C) are given in Supplementary file 4.

Figure 4—figure supplement 1. Model of Tom70’s role in the cellular response to the mitochondrial import defect.

(A) Schematic of mitochondrial import steps involve Hsp70 and Tom70. In yeast, Hsp70 binds and maintains nascent mitochondrial proteins in unfolded states before transferring them to Tom70 for import (Young, 2003). Tim23, the key subunits of the inner membrane translocase TIM complex, acts downstream of TOM complex (including the subunit Tom70) in mitochondrial import. Tim23 inactivation blocks the mitochondrial import of nascent mitochondrial proteins, which stay on the surface of mitochondria and associate with upstream factors (such as Tom70), activate the stress response to repress the biogenesis of mitochondrial proteins. (B) The inactivation of Tim23 blocks the mitochondrial import of nascent mitochondrial proteins, which occupy Tom70 and signal the nucleus to reduce the biogenesis of mitochondrial proteins. This allows the tim23^ts cells to re-balance the biogenesis and import of mitochondrial proteins, thereby avoiding their cytosolic accumulation and aggregation. Tom70 is required for this mitochondria-to-nucleus signaling as knockout of Tom70 in tim23^ts cells compromises this re-balancing effort and causes cytosolic protein aggregation. The fact that overexpression of TOM70, which activates the transcriptional activity of mitochondrial proteins, compromises the re-balancing effort of tim23^ts cells is consistent with previous reports that transcriptional repression of mitochondrial proteins is a key feature of the cellular response toward mitochondrial import defects .