The cytosolic cochaperone Sti1 is relevant for mitochondrial biogenesis and morphology

Abstract

Most mitochondrial proteins are synthesized in the cytosol prior to their import into the organelle. It is commonly accepted that cytosolic factors are required for delivering precursor proteins to the mitochondrial surface and for keeping newly synthesized proteins in an import-competent conformation. However, the identity of such factors and their defined contribution to the import process are mostly unknown. Using a presequence-containing model protein and a site-directed photo-crosslinking approach in yeast cells we identified the cytosolic chaperones Hsp70 (Ssa1) and Hsp90 (Hsp82) as well as their cochaperones, Sti1 and Ydj1, as putative cytosolic factors involved in mitochondrial protein import. Deletion of STI1 caused both alterations in mitochondrial morphology and lower steady-state levels of a subset of mitochondrial proteins. In addition, double deletion of STI1 with the mitochondrial import factors, MIM1 or TOM20, showed a synthetic growth phenotype indicating a genetic interaction of STI1 with these genes. Moreover, recombinant cytosolic domains of the import receptors Tom20 and Tom70 were able to bind in vitro Sti1 and other cytosolic factors. In summary, our observations point to a, direct or indirect, role of Sti1 for mitochondrial functionality.

Pex19 is involved in importing dually targeted tail‐anchored proteins to both mitochondria and peroxisomes

“…To investigate further the involvement of cytosolic factors in Fis1 biogenesis, the effect of deletion of the Hsp70 cochaperone Sti1 on mitochondrial levels of Fis1 was examined. Previously, we observed reduced Fis1 levels in whole cell lysate of sti1Δ cells . Analysis of mitochondria isolated from either sti1Δ or wild type (WT) cells revealed that at both normal (30°C) and elevated temperature (37°C) the levels of Fis1 are significantly reduced in the organelles from the mutated cells (Figure ).…”

“…The wild‐type strains BY4741 and W303a/α were utilized. The sti1Δ strain was described before, ssa2‐4Δ + SSA1 wt and ssa2‐4Δ + SSA1 ts strains are a kind gift from Dr O. Pines, and the BY4741 pex19Δ strain was obtained from Euroscarf. The PEX19 gene in the W303a background was deleted by the homologous recombination approach .…”

“…Mitochondria were isolated from yeast cells grown on sucrose‐containing media as described previously . Isolation of mitochondria‐enriched fractions was performed according to an established protocol . Prior to such isolation, Ssa2‐4Δ + SSA1 wt and Ssa2‐4Δ + SSA1 ts cells were grown at 24°C (permissive temperature) and shifted for 4 hours to 37°C (nonpermissive temperature).…”

mPOS is a novel mitochondrial trigger of cell death – implications for neurodegeneration

“…, pathway 7). The cytosolic Hsp70, Hsp90, and their cochaperones participate in the stabilization and delivery of precursors onto the OMM for import . Interestingly, the ribosome‐associated chaperones, Ssb1, Ssb2 and Zuo1, were found to suppress ρ o ‐lethality and mPOS‐induced cell death in tim18 ∆, yme1 ∆, tom70 ∆, atp1 ∆ and aac2 A128P cells, and loss of these chaperones renders the cells ρ o ‐lethal .…”

Escorted by chaperones: Sti1 helps to usher precursor proteins from the ribosome to mitochondria

“…The site‐specific binding of Sti1 to the presequence of a mitochondrial precursor protein reported by Hoseini et al . adds a critical player into the early reactions that mediate protein targeting to mitochondria which will be instrumental to fill the black box in our understanding of this important process.…”