ABSTRACT
Eukaryotic AAA proteases form a conserved family of membrane-embedded ATP-dependent proteases but have been analyzed functionally only in the yeast Saccharomyces cerevisiae. Here, we have identified two novel members of this protein family in the filamentous fungus Neurospora crassa, which were termed MAP-1 and IAP-1. Both proteins are localized to the inner membrane of mitochondria. They are part of two similar-sized high molecular mass complexes, but expose their catalytic sites to opposite membrane surfaces, namely, the intermembrane and the matrix space. Disruption of iap-1 by repeat-induced point mutation caused a slow growth phenotype at high temperature and stabilization of a misfolded inner membrane protein against degradation. IAP-1 could partially substitute for functions of its yeast homolog Yme1, demonstrating functional conservation. However, respiratory growth at 37 degrees C was not restored. Our results identify two components of the quality control system of the mitochondrial inner membrane in N. crassa and suggest that AAA proteases with catalytic sites exposed to opposite membrane surfaces are present in mitochondria of all eukaryotic cells.
Subject(s)
Intracellular Membranes/enzymology , Metalloendopeptidases/chemistry , Metalloendopeptidases/metabolism , Mitochondria/enzymology , Neurospora crassa/enzymology , ATP-Dependent Proteases , Adenosine Triphosphatases/genetics , Adenosine Triphosphatases/metabolism , Amino Acid Sequence , Catalytic Domain , Chromatography, Gel , Cloning, Molecular , Fungal Proteins/chemistry , Fungal Proteins/genetics , Fungal Proteins/metabolism , Genes, Fungal/genetics , Genetic Complementation Test , Intracellular Membranes/metabolism , Macromolecular Substances , Membrane Proteins/chemistry , Membrane Proteins/genetics , Membrane Proteins/metabolism , Metalloendopeptidases/genetics , Molecular Sequence Data , Molecular Weight , Mutation , Neurospora crassa/cytology , Protein Folding , Protein Subunits , Saccharomyces cerevisiae Proteins/genetics , Saccharomyces cerevisiae Proteins/metabolism , Sequence Homology, Amino Acid , Temperature , Time FactorsABSTRACT
An ubiquitous and conserved proteolytic system regulates the stability of mitochondrial inner membrane proteins. Two AAA proteases with catalytic sites at opposite membrane surfaces form a membrane-integrated quality control system and exert crucial functions during the biogenesis of mitochondria. Their activity is modulated by another membrane-protein complex that is composed of prohibitins. Peptides generated upon proteolysis in the matrix space are transported across the inner membrane by an ATP-binding cassette transporter. The function of these conserved components is discussed in the present review.
Subject(s)
Endopeptidases/metabolism , Membrane Proteins/metabolism , Mitochondria/physiology , Proteasome Endopeptidase Complex , Repressor Proteins , Chloroplasts/enzymology , Mitochondria/enzymology , Models, Biological , Peptide Hydrolases/metabolism , Prohibitins , Protein Subunits , Proteins/metabolism , Saccharomyces cerevisiae/enzymologyABSTRACT
Tcm62p, distantly related to chaperonins, is required for the assembly of succinate dehydrogenase in mitochondria of Saccharomyces cerevisiae and was proposed to exert chaperone activity. We demonstrate here crucial functions of Tcm62p under heat stress. It ensures mitochondrial gene expression at elevated temperatures and prevents heat-aggregation of the ribosomal subunit Var1p. Similar to chaperonins, Tcm62p forms a high molecular mass protein complex of approximately 850 kDa in the mitochondrial matrix space. These results suggest a more general chaperone function of Tcm62p in mitochondria.
