ABSTRACT
The majority of preproteins destined for mitochondria carry N-terminal presequences. The presequence translocase of the inner mitochondrial membrane (TIM23 complex) plays a central role in protein sorting. Preproteins are either translocated through the TIM23 complex into the matrix or are laterally released into the inner membrane. We report that the small hydrophobic protein Mgr2 controls the lateral release of preproteins. Mgr2 interacts with preproteins in transit through the TIM23 complex. Overexpression of Mgr2 delays preprotein release, whereas a lack of Mgr2 promotes preprotein sorting into the inner membrane. Preproteins with a defective inner membrane sorting signal are translocated into the matrix in wild-type mitochondria but are released into the inner membrane in Mgr2-deficient mitochondria. We conclude that Mgr2 functions as a lateral gatekeeper of the mitochondrial presequence translocase, providing quality control for the membrane sorting of preproteins.
Subject(s)
Membrane Proteins/metabolism , Membrane Transport Proteins/metabolism , Mitochondrial Membranes/metabolism , Saccharomyces cerevisiae Proteins/metabolism , Saccharomyces cerevisiae/metabolism , GTP-Binding Proteins/metabolism , Membrane Proteins/genetics , Methotrexate/pharmacology , Mitochondria/metabolism , Mitochondria/ultrastructure , Mitochondrial Precursor Protein Import Complex Proteins , Mitochondrial Proteins/metabolism , Protein Transport/drug effects , Saccharomyces cerevisiae Proteins/geneticsABSTRACT
The presequence translocase of the inner mitochondrial membrane (TIM23 complex) is essential for importing cleavable preproteins into mitochondria. The preproteins contain amino-terminal targeting sequences that are removed by the mitochondrial processing peptidase (MPP). Some preproteins carry bipartite presequences that are cleaved twice, by MPP and the inner membrane protease (IMP). Here, we report that the TIM23 complex is altered in mitochondria lacking the IMP subunit Imp1 although none of the TIM23 components contains a bipartite presequence. We show that the TIM23 subunit Mgr2 is processed by IMP, but not by MPP. The cytosolic precursor of Mgr2 contains a carboxy-terminal sequence that promotes targeting to mitochondria, but impairs stable assembly and function of the mature TIM23 complex. IMP removes the carboxy-terminal targeting sequence and thus promotes proper assembly of the TIM23 complex. Our results reveal carboxy-terminal processing as a new mechanism in the biogenesis of the mitochondrial inner membrane.
Subject(s)
Endopeptidases/metabolism , Membrane Proteins/chemistry , Membrane Proteins/metabolism , Membrane Transport Proteins/metabolism , Mitochondria/metabolism , Mitochondrial Proteins/metabolism , Saccharomyces cerevisiae Proteins/chemistry , Saccharomyces cerevisiae Proteins/metabolism , Saccharomyces cerevisiae/enzymology , Amino Acid Motifs , Endopeptidases/genetics , Membrane Proteins/genetics , Membrane Transport Proteins/chemistry , Membrane Transport Proteins/genetics , Mitochondria/chemistry , Mitochondria/enzymology , Mitochondria/genetics , Mitochondrial Membranes/chemistry , Mitochondrial Membranes/metabolism , Mitochondrial Precursor Protein Import Complex Proteins , Mitochondrial Proteins/genetics , Protein Multimerization , Protein Processing, Post-Translational , Protein Transport , Saccharomyces cerevisiae/chemistry , Saccharomyces cerevisiae/genetics , Saccharomyces cerevisiae/metabolism , Saccharomyces cerevisiae Proteins/geneticsABSTRACT
Many mitochondrial proteins are synthesized with N-terminal presequences in the cytosol. The presequence translocase of the inner mitochondrial membrane (TIM23) translocates preproteins into and across the membrane and associates with the matrix-localized import motor. The TIM23 complex consists of three core components and Tim21, which interacts with the translocase of the outer membrane (TOM) and the respiratory chain. We have identified a new subunit of the TIM23 complex, the inner membrane protein Mgr2. Mitochondria lacking Mgr2 were deficient in the Tim21-containing sorting form of the TIM23 complex. Mgr2 was required for binding of Tim21 to TIM23(CORE), revealing a binding chain of TIM23(CORE)-Mgr2/Tim21-respiratory chain. Mgr2-deficient yeast cells were defective in growth at elevated temperature, and the mitochondria were impaired in TOM-TIM23 coupling and the import of presequence-carrying preproteins. We conclude that Mgr2 is a coupling factor of the presequence translocase crucial for cell growth at elevated temperature and for efficient protein import.