Immunological detection of the mitochondrial F1-ATPase alpha subunit in the matrix of rat liver peroxisomes. A protein involved in organelle biogenesis?

Analysis of the golden Syrian hamster anterior pituitary gland proteome by ion trap mass spectrometry

We utilized mass spectrometry (MS) and bioinformatics to investigate the proteome of the anterior pituitary gland (AP). Subcellular fractions of APs from 2-month-old male Golden Syrian hamsters were prepared for protein denaturation, treatment with trypsin and analyses utilizing micro liquid chromatography MS/MS and the database search software SEQUEST. In the nuclear, non-nuclear 100,000 x g and cytosolic fractions we identified 76, 52 and 52 different proteins, respectively. A total of 145 distinct proteins were detected. We identified growth hormone, prolactin, pro-opiomelanocortin, the alpha-subunit for the glycoprotein hormones, luteinizing hormone-beta and follicle-stimulating hormone-beta. Groups of other identified proteins included hormone processing, secretion granule associated, non-hormonal endoplasmic reticulum associated, calcium binding, protein kinase C associated histone and non-histone chromosomal material, other RNA-binding, splicing factors, heterogeneous nuclear ribonucleoproteins, helicases, lamins, microfilament associated, microtubule associated, adenosine triphosphate and guanosine diphosphate associated, keratins, lysosomal, ribosomal, enzymes in glycolysis and the tricarboxylic and pentose phosphate paths, glutathione associated, transmethylation, catabolic and unknown protein products as well as blood hemoglobins. Proteins previously not reported in the AP, such as fertility protein SP22, were identified. The proteins identified in the present study form a foundation for defining the proteome in normal adult male AP.

Association of ATP synthase alpha-chain with neurofibrillary degeneration in Alzheimer's disease

Amyloid deposits and neurofibrillary tangles (NFT) are the two hallmarks that characterize Alzheimer's disease (AD). In order to find the molecular partners of these degenerating processes, we have developed antibodies against insoluble AD brain lesions. One clone, named AD46, detects only NFT. Biochemical and histochemistry analyses demonstrate that the labeled protein accumulating in the cytosol of Alzheimer degenerating neurons is the alpha-chain of the ATP synthase. The cytosolic accumulation of the alpha-chain of ATP synthase is observed even at early stages of neurofibrillary degenerating process. It is specifically observed in degenerating neurons, either alone or tightly associated with aggregates of tau proteins, suggesting that it is a new molecular event related to neurodegeneration. Overall, our results strongly suggest the implication of the alpha-chain of ATP synthase in neurofibrillary degeneration of AD that is illustrated by the cytosolic accumulation of this mitochondrial protein, which belongs to the mitochondrial respiratory system. This regulatory subunit of the respiratory complex V of mitochondria is thus a potential target for therapeutic and diagnostic strategies.

Molecular cloning of a peroxisomal Ca2+-dependent member of the mitochondrial carrier superfamily

A cDNA from a novel Ca2+-dependent member of the mitochondrial solute carrier superfamily was isolated from a rabbit small intestinal cDNA library. The full-length cDNA clone was 3,298 nt long and coded for a protein of 475 amino acids, with four elongation factor-hand motifs located in the N-terminal half of the molecule. The 25-kDa N-terminal polypeptide was expressed in Escherichia coli, and it was demonstrated that it bound Ca2+, undergoing a reversible and specific conformational change as a result. The conformation of the polypeptide was sensitive to Ca2+ which was bound with high affinity (Kd approximately 0.37 microM), the apparent Hill coefficient for Ca2+-induced changes being about 2.0. The deduced amino acid sequence of the C-terminal half of the molecule revealed 78% homology to Grave disease carrier protein and 67% homology to human ADP/ATP translocase; this sequence homology identified the protein as a new member of the mitochondrial transporter superfamily. Northern blot analysis revealed the presence of a single transcript of about 3,500 bases, and low expression of the transporter could be detected in the kidney but none in the liver. The main site of expression was the colon with smaller amounts found in the small intestine proximal to the ileum. Immunoelectron microscopy localized the transporter in the peroxisome, although a minor fraction was found in the mitochondria. The Ca2+ binding N-terminal half of the transporter faces the cytosol.

Variable peroxisomal and mitochondrial targeting of alanine: glyoxylate aminotransferase in mammalian evolution and disease

Under the putative influence of dietary selection pressure, the subcellular distribution of alanine:glyoxylate aminotransferase 1 (AGT) has changed on many occasions during the evolution of mammals. Depending on the particular species, AGT can be found either in peroxisomes or mitochondria, or in both peroxisomes and mitochondria. This variable localization depends on the differential expression of N-terminal mitochondrial and C-terminal peroxisomal targeting sequences by the use of alternative transcription and translation initiation sites. AGT is peroxisomal in most humans, but it is mistargeted to the mitochondria in a subset of patients suffering from the rare hereditary disease primary hyperoxaluria type 1. Mistargeting is due to the unlikely combination of a normally occurring polymorphism that generates a functionally weak mitochondrial targeting sequence and a disease-specific mutation which, in combination with the polymorphism, inhibits AGT dimerization. The mechanisms by which AGT can be targeted differentially to peroxisomes and/or mitochondria highlight the different molecular requirements for protein import into these two organelles.

mRNA encoding the beta-subunit of the mitochondrial F1-ATPase complex is a localized mRNA in rat hepatocytes

Subcellular mRNA localization has emerged as a mechanism for regulation of gene expression and protein-sorting pathways. Here we describe the different cytoplasmic presentation in rat hepatocytes of two nuclear mRNA species encoding subunits alpha and beta of the mitochondrial F1-ATPase complex. alpha-F1-ATPase mRNA is dispersed and scattered in the cytoplasm. In contrast, beta-F1-ATPase mRNA appears in rounded electron-dense clusters, often in close proximity to mitochondria. Hybridization experiments with beta2-microglobulin and beta-actin cDNA species reveal an expected subcellular distribution pattern of the mRNA species and a non-clustered appearance. Development does not alter the presentation of beta-F1-ATPase mRNA hybrids, although it affects the relative abundance of beta-F1-ATPase mRNA clusters in the cytoplasm of the hepatocyte. These findings illustrate in vivo the existence of two different sorting pathways for the nuclear-encoded mRNA species of mitochondrial proteins. High-resolution immunocytochemistry and immunoprecipitation experiments allowed the identification of the beta-subunit precursor in the cytoplasm of the hepatocyte, also suggesting a post-translational import pathway for this precursor protein. It is suggested that the localization of beta-F1-ATPase mRNA in a subcellular structure of the hepatocyte might have implications for the control of gene expression at post-transcriptional levels during mitochondrial biogenesis in mammals.

Cpn60 is exclusively localized into mitochondria of rat liver and embryonic Drosophila cells

Several reports have claimed that the mitochondrial chaperonin cpn60, or a close homolog, is also present in some other subcellular compartments of the eukaryotic cell. Immunoelectron microscopy studies, using a polyclonal serum against cpn60, revealed that the protein is exclusively localized within the mitochondria of rat liver and embryonic Drosophila cells (SL2). Furthermore, no cpn60 immunoreactive material could be found within the nucleus of SL2 cells subjected to a 1 h 37 degrees C heat-shock treatment. In contrast to these findings, immunoelectron microscopy studies, using a cpn60 monoclonal antibody, revealed mitochondrial and extramitochondrial (plasma membrane, nucleus) immunoreactive material in rat liver cells. Surprisingly, the monoclonal antibody also reacted with fixed proteins of the mature red blood cell. The monoclonal antibody, as well as cpn60 polyclonal sera, only recognize mitochondrial cpn60 in Western blots of liver proteins. Furthermore, none of the cpn60 antibodies used in this study recognized blotted proteins from rat red blood cells. Therefore, we suggest that the reported extramitochondrial localization of cpn60 in metazoan cells may be due to cross-reactivity of some of cpn60 antibodies with conformational epitopes also present in distantly related cpn60 protein homologs that are preserved during fixation procedures of the cells.

Gene structure and cell type-specific expression of the human ATP synthase alpha subunit

The gene structure of the human ATP synthase alpha subunit (hATP1) was determined by cloning and sequencing. This gene is approximately 14 kbp in length and contains 12 exons interrupted by 11 introns. Mapping of the clones of hATP1 and Southern blot analysis of the genomic gene showed that there were a single copy of bona fide hATP1 gene and two pseudogenes. Primer extension and S1 mapping analysis showed the presence of multiple transcription initiation sites of the hATP1 gene. No TATA box or CAAT box was found near the transcription initiation sites. Comparison with the bovine gene showed that the 5'-flanking region of the hATP1 gene has an unconserved guanine-cytosine (GC) rich region, including several binding motifs of transcriptional factors, such as Sp1, AP-2, and GCF. By functional assay of gene expression, the basal promoter activity was located near the GC rich region. Comparison of the 5'-upstream region of the hATP1 gene with those of the genes for bovine ATP synthase alpha, human beta, and human gamma subunits indicated three common sequences, suggesting that putative cis-elements coordinate the expressions of the three subunit genes for the ATP synthase. The enhancer activities derived from the 5'-deletion mutants of a hATP1-CAT chimeric gene were different in cell lines from four different human tissues, suggesting the existence of cell type-specific gene regulation.

Import of proteins into peroxisomes and other microbodies

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