Biomedical subjects
Publications and source records attributed to M Meijer.
Translocation of precursor proteins into mitochondria depends on loosely assembled protein complexes in the outer and inner membranes. Recent studies indicate that dynamic interactions of subcomplexes and cooperation with molecular chaperones drive key steps in protein import.
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The preprotein translocase of the inner mitochondrial membrane has only been described in Saccharomyces cerevisiae to date. We report that the essential subunit Tim17 is highly conserved in evolution. The targeting and assembly of yeast Tim17 as well as that of human and Drosophila melanogaster Tim17 were characterized with isolated yeast mitochondria. Targeting signals in the mature protein direct the Tim17 precursors to the receptor Tom70 on the mitochondrial surface. In a membrane potential-dependent step the precursors insert into the inner membrane, adopt a characteristic topology and assemble with Tim23. The mechanisms of targeting and assembly were indistinguishable between the Tim17s from distinct organisms, indicating a high evolutionary conservation.
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The essential mitochondrial inner-membrane protein, Mim44, is involved in the translocation of preproteins across the mitochondrial inner membrane. Two other putative components of this protein-translocation system are the integral inner-membrane proteins, Mim23 and Mim17. Here, we present genetic evidence for functional co-operation of all three proteins. Furthermore, we show that Mim23 and Mim17 are associated in a protein complex that also contains two proteins of 55 kDa and 20 kDa. We speculate that this subcomplex forms the proteinaceous import channel of the inner-membrane which transiently interacts with a less abundant peripheral complex of Mim44 and mitochondrial heat-shock protein Hsp70.
Two driving mechanisms, one powered by membrane potential and the other a combination of pulling and trapping, seem to be involved in protein import into mitochondria.
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We analyzed four Saccharomyces cerevisiae mutants defective in mitochondrial protein import and found that they are complemented by a novel gene encoding a 17 kDa protein. The protein is integrally located in the mitochondrial inner membrane and is termed MIM17. It shows significant homology to MIM23/Mas6p, a previously identified mitochondrial inner membrane protein required for the import of preproteins. Like MIM23, the precursor of MIM17 is synthesized without a presequence. A deletion of MIM17 is lethal. MIM17 thus joins the small group of mitochondrial proteins that are essential for the viability of yeast. We propose that MIM17 is an essential component of the preprotein import machinery of the mitochondrial inner membrane.
Three proteins of the mitochondrial inner membrane are known that are essential for the viability of yeast and seem to be involved in import of preproteins; the integral membrane proteins MIM17 and MIM23 and the peripheral membrane protein MIM44, MIM17 and MIM23 are homologous to each other in their hydrophobic domain, expose their termini to the intermembrane space, and span the inner membrane up to four times, each. A preprotein in transit across the mitochondrial membrane is specifically cross-linked to MIM17, MIM23, MIM44, and matrix hsp70. We conclude that MIM17 and MIM23 are integral parts of a preprotein translocation channel and cooperate with MIM44 and hsp70 at the same protein import site.
Mitochondria import most of their proteins from the cytosol. Although considerable information is available on the import machineries of the mitochondrial outer membrane and matrix, until recently little was known about the machinery of the inner membrane. Recent studies have identified three mitochondrial inner membrane proteins (MIMs) as essential components of the import machinery. MIM17 and MIM23 seem to form part of a channel, while MIM44, in cooperation with the heat-shock protein Hsp70, binds the preproteins in transit. The electrical membrane potential and ATP are needed to drive protein translocation through the MIM import machinery.
The import of preproteins into mitochondria involves translocation of the polypeptide chains through putative channels in the outer and inner membranes. Preprotein-binding proteins are needed to drive the unidirectional translocation of the precursor polypeptides. Two of these preprotein-binding proteins are the peripheral inner membrane protein MIM44 and the matrix heat shock protein hsp70. We report here that MIM44 is mainly exposed on the matrix side, and a fraction of mt-hsp70 is reversibly bound to the inner membrane. Mt-hsp70 binds to MIM44 in a 1:1 ratio, suggesting that mt-hsp70 is localizing to the membrane via its interaction with MIM44. Formation of the complex requires a functional ATPase domain of mt-hsp70. Addition of Mg-ATP leads to dissociation of the complex. Overexpression of mt-hsp70 rescues the protein import defect of mutants in MIM44; conversely, overexpression of MIM44 rescues protein import defects of mt-hsp70 mutants. In addition, yeast strains with conditional mutations in both MIM44 and mt-hsp70 are barely viable, showing a synthetic growth defect compared to strains carrying single mutations. We propose that MIM44 and mt-hsp70 cooperate in translocation of preproteins. By binding to MIM44, mt-hsp70 is recruited at the protein import sites of the inner membrane, and preproteins arriving at MIM44 may be directly handed over to mt-hsp70.
This review on Brucella abortus in cattle covers the pathogenesis, the epidemiology and the diagnostics of brucellosis. Emphasis is given to the presence of latent infections in young stock. Calves infected by B. abortus in utero or after ingestion of infected milk may acquire a persistent infection. These animals might present a significant problem in brucellosis control and eradication schemes, since they are difficult to detect by the usual serological tests as they remain negative until near the first calving or abortion. The diagnostics must be improved: (new) tests need to be made more sensitive and herds and/or animals should be tested more frequently after introduction of cattle into a herd. Moreover more attention should be paid to cases of abortion. It is also suggested that if the slaughter of infected herds is limited to adult animals, the heifer calves could be a source of infection to the restocked herd.
A screening for yeast mutants impaired in mitochondrial protein import led to the identification of two genes (MPII and MPI2) encoding the essential components MIM44 and MIM17 of the inner membrane import machinery. We analyzed twelve additional mutants obtained in the screening and found two further complementation groups. One group represents mutants of SSC1, the gene encoding mitochondrial hsp70, an essential matrix protein required for protein import across the inner membrane. The second complementation group represents mutants of a new gene (MP13) encoding a 23 kDa integral inner membrane protein (MIM23). MIM23 is synthesized without a presequence, and its import to the inner membrane requires a membrane potential. MIM23 contains a domain homologous to half of MIM17. We speculate that MIM23 is a new member of the protein import machinery of the mitochondrial inner membrane.
The essential yeast gene MPI1 encodes a mitochondrial membrane protein that is possibly involved in protein import into the organelle (A. C. Maarse, J. Blom, L. A. Grivell, and M. Meijer, EMBO J. 11:3619-3628, 1992). For this report, we determined the submitochondrial location of the MPI1 gene product and investigated whether it plays a direct role in the translocation of preproteins. By fractionation of mitochondria, the mature protein of 44 kDa was localized to the mitochondrial inner membrane and therefore termed MIM44. Import of the precursor of MIM44 required a membrane potential across the inner membrane and involved proteolytic processing of the precursor. A preprotein in transit across the mitochondrial membranes was cross-linked to MIM44, whereas preproteins arrested on the mitochondrial surface or fully imported proteins were not cross-linked. When preproteins were arrested at two distinct stages of translocation across the inner membrane, only preproteins at an early stage of translocation could be cross-linked to MIM44. Moreover, solubilized MIM44 was found to interact with in vitro-synthesized preproteins. We conclude that MIM44 is a component of the mitochondrial inner membrane import machinery and interacts with preproteins in an early step of translocation.
A study was performed of the course of major depressions (DSM-III) of elderly people who had been admitted to a psychogeriatric ward of a general psychiatric hospital. Patients were selected by searching the files. Patients with another diagnosis on axis I or a serious physical illness were excluded. The selected patients were traced and asked to participate in a follow-up investigation. From the files 38 patients were selected, of whom three had died in hospital. The remaining 35 patients included 28 women and seven men, with an average age of 78 years. After discharge seven of them died a natural death and six patients refused to participate. The remaining 22 patients were visited. From the files it appeared that 54% had completely recovered at discharge. Patients with a delusional depression had been hospitalised significantly longer, had been treated with more medicines and had less often been completely recovered at discharge. At the time of the follow up 64% had completely recovered but 32% had had a relapse. Patients with a delusional depression less often recovered completely. A remarkably low percentage of the population studied were able to live on their own. The results of this study are compared with foreign studies.
To identify components of the mitochondrial protein import pathway in yeast, we have adopted a positive selection procedure for isolating mutants disturbed in protein import. We have cloned and sequenced a gene, termed MPI1, that can rescue the genetic defect of one group of these mutants. MPI1 encodes a hydrophilic 48.8 kDa protein that is essential for cell viability. Mpi1p is a low abundance and constitutively expressed mitochondrial protein. Mpi1p is synthesized with a characteristic mitochondrial targeting sequence at its amino-terminus, which is most probably proteolytically removed during import. It is a membrane protein, oriented with its carboxy-terminus facing the intermembrane space. In cells depleted of Mpi1p activity, import of the precursor proteins that we tested thus far, is arrested. We speculate that the Mpi1 protein is a component of a proteinaceous import channel for translocation of precursor proteins across the mitochondrial inner membrane.
Until recently, research on borderline disorder in children has sought the common denominator of the symptoms. In recent years there have been attempts to circumscribe the definition with the help of DSM-III criteria and the DIB. This approach appears fruitful. The scanty data on schizotypal children suggest that the validity of this diagnosis in childhood should be investigated. In adolescence it is possible to discern those with borderline and schizotypal disorders whose symptoms meet both DIB and DSM-III-R criteria respectively. No data exist, however, concerning the predictive validity of such disorders in adolescents. Classification on an empirical basis is advocated in order to refine the diagnosis of these and related disorders in children and adolescents.
Biosynthesis of a functional mitochondrion requires the coordinate expression of genes in both mitochondrial and nuclear DNAs. In yeast, three mitochondrial genes are split and RNA splicing plays a pivotal role in their expression. The recent finding that some introns are capable of self-splicing activity in vitro has permitted analysis of the mechanisms involved in RNA catalysis and may eventually shed light on the evolution of splicing mechanisms in general. Most mitochondrial proteins are encoded by nuclear genes, synthesized in the cytoplasm and imported by the organelle. The availability of cloned genes coding for several constituent subunits of the ubiquinol-cytochrome c reductase, which are imported by mitochondria, has allowed study of selected steps in the addressing of proteins to mitochondria and their intercompartmental sorting within the organelle. Recent developments are discussed.