The antiapoptotic OPA1/Parl couple participates in mitochondrial adaptation to heat shock.

The mitochondria-shaping protein optic atrophy 1 (OPA1) has genetically distinguishable roles in mitochondrial morphology and apoptosis. The latter depends on the presenilin associated rhomboid like (PARL) protease, essential for the accumulation of a soluble intermembrane space form of OPA1 (IMS-OPA1). Here we show that OPA1 and PARL participate in the heat shock response, a stereotypical cellular process of adaptation to thermal stress. Upon heat shock, long forms of OPA1 are lost and mitochondria fragment. However, mitochondrial fusion is dispensable to maintain viability, whereas IMS-OPA1 is required. Upon conditioning-a process of mild heat shock and recovery-IMS-OPA1 accumulates, OPA1 oligomers increase and mitochondria release less cytochrome c, ultimately resulting in cellular resistance to subsequent apoptotic inducers. In Parl(-/-) cells accumulation of IMS-OPA1 is blunted and conditioning fails to protect from cytochrome c release and apoptosis. Thus, the OPA1/PARL dependent pathway of cristae remodeling is implicated in heat shock. This article is part of a Special Issue entitled: 17th European Bioenergetics Conference (EBEC 2012).

Preprotein transport machineries of yeast mitochondrial outer membrane are not required for Bax-induced release of intermembrane space proteins.

The mitochondrial outer membrane contains protein import machineries, the translocase of the outer membrane (TOM) and the sorting and assembly machinery (SAM). It has been speculated that TOM or SAM are required for Bax-induced release of intermembrane space (IMS) proteins; however, experimental evidence has been scarce. We used isolated yeast mitochondria as a model system and report that Bax promoted an efficient release of soluble IMS proteins while preproteins were still imported, excluding an unspecific damage of mitochondria. Removal of import receptors by protease treatment did not inhibit the release of IMS proteins by Bax. Yeast mutants of each Tom receptor and the Tom40 channel were not impaired in Bax-induced protein release. We analyzed a large collection of mutants of mitochondrial outer membrane proteins, including SAM, fusion and fission components, but none of these components was required for Bax-induced protein release. The released proteins included complexes up to a size of 230 kDa. We conclude that Bax promotes efficient release of IMS proteins through the outer membrane of yeast mitochondria while the inner membrane remains intact. Inactivation of the known protein import and sorting machineries of the outer membrane does not impair the function of Bax at the mitochondria.

Inactivation of the mitochondrial heat shock protein zim17 leads to aggregation of matrix hsp70s followed by pleiotropic effects on morphology and protein biogenesis.

The biogenesis of mitochondrial matrix proteins involves the translocase of the outer membrane, the presequence translocase of the inner membrane and the presequence translocase-associated motor. The mitochondrial heat shock protein 70 (mtHsp70) forms the central core of the motor. Recent studies led to the identification of Zim17, a mitochondrial zinc finger motif protein that interacts with mtHsp70. Different views have been reported on the localization of Zim17 in the mitochondrial inner membrane or matrix. Depletion of Zim17 impairs several critical mitochondrial processes, leading to inhibition of protein import, defects of Fe/S protein biogenesis and aggregation of Hsp70s in the matrix. Additionally, we found that inactivation of Zim17 altered the morphology of mitochondria. These pleiotropic effects raise the question of the specific function of Zim17 in mitochondria. Here, we report that Zim17 is a heat shock protein of the mitochondrial matrix that is loosely associated with the inner membrane. To address the function of Zim17 in organello, we generated a temperature-sensitive mutant allele of the ZIM17 gene in yeast. Upon a short-term shift of the yeast mutant cells to a non-permissive temperature, matrix Hsp70s aggregated while protein import, Fe/S protein activity and mitochondrial morphology were not, or only mildly, affected. Only after a long-term shift to non-permissive temperature, were strong defects in protein import, Fe/S protein activity and mitochondrial morphology observed. These findings suggest that the heat shock protein Zim17 plays a specific role in preventing protein aggregation in the mitochondrial matrix, and that aggregation of Hsp70s causes pleiotropic effects on protein biogenesis and mitochondrial morphology.

Mitochondrial F1Fo-ATP synthase: the small subunits e and g associate with monomeric complexes to trigger dimerization.

Mitochondrial F(1)F(o)-ATP synthase catalyzes the formation of ATP from ADP and inorganic phosphate. The enzyme is found in monomeric, dimeric and higher oligomeric forms in the inner mitochondrial membrane. Dimerization of ATP synthase complexes is a prerequisite for the generation of larger oligomers that promote membrane bending and formation of tubular cristae membranes. Two small proteins of the membrane-embedded F(o)-domain, subunit e (Su e; Atp21) and Su g (Atp20), were identified as dimer-specific subunits of yeast ATP synthase and shown to be required for stabilization of the dimers. We have identified two distinct monomeric forms of yeast ATP synthase. Su e and Su g are present not only in the dimer but also in one of the monomeric forms. We demonstrate that Su e and Su g sequentially assemble with monomeric ATP synthase to form a dimerization-competent primed monomer. We conclude that association of Su e and Su g with monomeric F(1)F(o)-ATP synthase represents an initial step of oligomer formation.

Essential role of Mia40 in import and assembly of mitochondrial intermembrane space proteins.

Mitochondria import nuclear-encoded precursor proteins to four different subcompartments. Specific import machineries have been identified that direct the precursor proteins to the mitochondrial outer membrane, inner membrane or matrix, respectively. However, a machinery dedicated to the import of mitochondrial intermembrane space (IMS) proteins has not been found so far. We have identified the essential IMS protein Mia40 (encoded by the Saccharomyces cerevisiae open reading frame YKL195w). Mitochondria with a mutant form of Mia40 are selectively inhibited in the import of several small IMS proteins, including the essential proteins Tim9 and Tim10. The import of proteins to the other mitochondrial subcompartments does not depend on functional Mia40. The binding of small Tim proteins to Mia40 is crucial for their transport across the outer membrane and represents an initial step in their assembly into IMS complexes. We conclude that Mia40 is a central component of the protein import and assembly machinery of the mitochondrial IMS.