The mitochondrial morphology protein Mdm10 functions in assembly of the preprotein translocase of the outer membrane.

The biogenesis of mitochondrial outer membrane proteins involves the general translocase of the outer membrane (TOM complex) and the sorting and assembly machinery (SAM complex). The two known subunits of the SAM complex, Mas37 and Sam50, are required for assembly of the abundant outer membrane proteins porin and Tom40. We have identified an unexpected subunit of the SAM complex, Mdm10, which is involved in maintenance of mitochondrial morphology. Mitochondria lacking Mdm10 are selectively impaired in the final steps of the assembly pathway of Tom40, including the association of Tom40 with the receptor Tom22 and small Tom proteins, while the biogenesis of porin is not affected. Yeast mutants of TOM40, MAS37, and SAM50 also show aberrant mitochondrial morphology. We conclude that Mdm10 plays a specific role in the biogenesis of the TOM complex, indicating a connection between the mitochondrial protein assembly apparatus and the machinery for maintenance of mitochondrial morphology.

Towards multidimensional liquid chromatography separation of proteins using fluorescence and isotope-coded protein labelling for quantitative proteomics.

HPLC has emerged as a valuable tool for separating proteins. To address the analysis of complex proteomes and quantitative changes of proteins therein, we developed a multidimensional LC (MDLC)-based approach followed by large gel 1-D SDS-PAGE. Here we present a novel strategy that allows for simultaneously identifying and quantifying differentially regulated proteins following three separation and fractionation steps. This MDLC platform integrates advantages of dual protein labelling using both fluorescence and isotope-coded tags for subsequent detection and quantification of abundance ratios of proteins by MS.

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.

Multidimensional chromatography: a powerful tool for the analysis of membrane proteins in mouse brain.

Understanding the function of membrane proteins is of fundamental importance due to their crucial roles in many cellular processes and their direct association with human disorders. However, their analysis poses a special challenge, largely due to their highly amphipathic nature. Until recently, analyses of proteomic samples mainly were performed by two-dimensional polyacrylamide gel electrophoresis (2D-PAGE), due to the unprecedented separation power of the technique. However, in conventional 2D-PAGE membrane proteins are generally underrepresented due to their tendency to precipitate during isoelectric focusing and their inefficient transfer from the first to the second dimension. As a consequence, several other separation techniques, primarily based on liquid chromatography (LC), have been employed for analysis of this group of proteins. In the present study, different LC-based methods were compared for the analysis of crude protein extracts. One- and two-dimensional high-performance liquid chromatographic (1D- and 2D-HPLC) separations of brain protein tryptic digests with a predicted concentration range of up to 5 orders of magnitude were found to be insufficient, thus making a preceding fractionation step necessary. An additional protein separation step was introduced and a 3D-PAGE-HPLC analysis was performed. The results of these experiments are compared with results of 2D-PAGE/matrix-assisted laser desorption ionization mass spectrometric (MALDI MS) analyses of the same samples. Features, challenges, advantages, and disadvantages of the respective systems are discussed. The brain (mouse and human) was chosen as the analyzed tissue as it is of high interest in medical and pharmaceutical research into neurological diseases such as multiple sclerosis, stroke, Alzheimer's disease, and Parkinson's disease. The study is part of our ongoing research aimed at identifying new biomarkers for neurodegenerative diseases.

Essential role of Isd11 in mitochondrial iron-sulfur cluster synthesis on Isu scaffold proteins.

Mitochondria are indispensable for cell viability; however, major mitochondrial functions including citric acid cycle and oxidative phosphorylation are dispensable. Most known essential mitochondrial proteins are involved in preprotein import and assembly, while the only known essential biosynthetic process performed by mitochondria is the biogenesis of iron-sulfur clusters (ISC). The components of the mitochondrial ISC-assembly machinery are derived from the prokaryotic ISC-assembly machinery. We have identified an essential mitochondrial matrix protein, Isd11 (YER048w-a), that is found in eukaryotes only. Isd11 is required for biogenesis of cellular Fe/S proteins and thus is a novel subunit of the mitochondrial ISC-assembly machinery. It forms a complex with the cysteine desulfurase Nfs1 and is required for formation of an Fe/S cluster on the Isu scaffold proteins. We conclude that Isd11 is an indispensable eukaryotic component of the mitochondrial machinery for biogenesis of Fe/S proteins.

The mono-ADP-ribosyltransferases Alt and ModB of bacteriophage T4: target proteins identified.

Infection of Escherichia coli by bacteriophage T4 leads to the expression of three phage mono-ADP-ribosyltransferases (namely, Alt, ModA, and ModB), each of which modifies a distinct group of host proteins. To improve understanding of these interactions and their consequences for the T4 replication cycle, we used high-resolution two-dimensional gel electrophoresis and mass-spectrometry to identify some of the putative target proteins ADP-ribosylated in vitro by Alt (total approximately 27) and ModB (total approximately 8). E. coli trigger factor and the elongation factor EF-Tu were 2 targets of ModB action, and these proteins were among the 10 identified as targets of Alt, hinting that these factors are involved in phage replication.

5th HUPO BPP Bioinformatics Meeting at the European Bioinformatics Institute in Hinxton, UK--Setting the analysis frame.

The Bioinformatics Committee of the HUPO Brain Proteome Project (HUPO BPP) meets regularly to execute the post-lab analyses of the data produced in the HUPO BPP pilot studies. On July 7, 2005 the members came together for the 5th time at the European Bioinformatics Institute (EBI) in Hinxton, UK, hosted by Rolf Apweiler. As a main result, the parameter set of the semi-automated data re-analysis of MS/MS spectra has been elaborated and the subsequent work steps have been defined.

Presence of detergent-resistant microdomains in lysosomal membranes.

We examined the association of acetyl-CoA:alpha-glucosaminide N-acetyltransferase, a lysosomal enzyme participating in the degradation of heparan sulfate with other components of the lysosomal membrane. We prepared lysosomal membranes from human placenta and treated them with zwitterionic and non-ionic detergents. Membrane proteins were solubilized either in the presence of CHAPS at room temperature or of Triton X-100 at 4 degrees C. The CHAPS-containing extract was subjected to gel filtration in a column with the nominal size exclusion of 0.6 MDa. Under these conditions the enzyme fractionated near the void volume. To examine the association of the enzyme with detergent-resistant lipid microdomains, the extract that had been prepared with Triton X-100 was subjected to flotation in a density gradient medium. After centrifugation, a major portion of the activity of the acetyltransferase was found at the top of the gradient along with the bulk of alkaline phosphatase. Alkaline phosphatase is a glycosylphosphatidylinositol-anchored protein; possibly a contaminant in the lysosomal fraction originating from the plasma membrane and adventitiously an internal control for the flotation in the gradient. In contrast, acetyltransferase is a genuine lysosomal protein that obligatorily spans the membrane since it transfers acetyl residues from acetyl-CoA in cytosol to glucosaminyl residues in heparan sulfate fragments in the lysosomal matrix. To our knowledge this is the first report on association of a lysosomal membrane protein with detergent-resistant membrane microdomains or rafts.

Oxygen tension regulates the expression of a group of procollagen hydroxylases.

In this study, we have characterized the influence of hypoxia on the expression of hydroxylases crucially involved in collagen fiber formation, such as prolyl-4-hydroxylases (Ph4) and procollagen lysyl-hydroxylases (PLOD). Using the rat vascular smooth muscle cell line A7r5, we found that an hypoxic atmosphere caused a characteristic time-dependent five- to 12-fold up-regulation of the mRNAs of the two P4h alpha-subunits [alphaI (P4ha1) and alphaII (P4ha2)] and of two lysylhydroxylases (PLOD1 and PLOD2). These effects of hypoxia were mimicked by the iron-chelator deferoxamine (100 micro m) and by cobaltous chloride (100 micro m). The hypoxic induction of these genes was also seen in the mouse juxtaglomerular As4.1 cell line and mouse hepatoma cell line Hepa1 but was almost absent in the mutant cell line Hepa1C4, which is defective for the hypoxia-inducible transcription factor 1 (HIF-1). In addition, the enzyme expression was induced by hypoxia in mouse embryonic fibroblasts but not in embryonic fibroblasts lacking the HIF-1alpha subunit. These findings indicate that hypoxia stimulates the gene expression of a cluster of hydroxylases that are indispensible for collagen fiber formation. Strong indirect evidence, moreover, suggests that the expression of these enzymes during hypoxia is coordinated by HIF-1.

Sam35 of the mitochondrial protein sorting and assembly machinery is a peripheral outer membrane protein essential for cell viability.

The mitochondrial outer membrane contains two integral proteins essential for cell viability, Tom40 of the translocase of the outer membrane (TOM complex) and Sam50 of the sorting and assembly machinery (SAM complex). Here we report the identification of Sam35, the first peripheral mitochondrial outer membrane protein that is essential for cell viability. Sam35 (encoded by the Saccharomyces cerevisiae ORF YHR083w) is a novel subunit of the SAM complex and is crucial for the assembly pathway of outer membrane beta-barrel proteins, such as the precursors of Tom40 and porin. Sam35 is not required for the import of inner membrane or matrix targeted proteins. The presence of two essential proteins in the SAM complex, Sam35 and Sam50, indicates that it plays a central role in mitochondrial biogenesis.

The cellular oxygen tension regulates expression of the endoplasmic oxidoreductase ERO1-Lalpha.

The formation of disulfide bonds in the endoplasmic reticulum requires protein disulfide isomerase (PDI) and endoplasmic reticulum oxidoreductin 1 (ERO1) that reoxidizes PDI. We report here that the expression of the rat, mouse and human homologues of ERO1-Like protein alpha but not of the isoform ERO1-Lbeta are stimulated by hypoxia in rats vivo and in rat, mouse and human cell cultures. The temporal pattern of hypoxic ERO1-Lalpha induction is very similar to that of genes triggered by the hypoxia inducible transcription factor (HIF-1) and is characteristically mimicked by cobalt and by deferoxamine, but is absent in cells with a defective aryl hydrocarbon receptor translocator (ARNT, HIF-1beta). We speculate from these findings that the expression of ERO1-Lalpha is probably regulated via the HIF-pathway and thus belongs to the family of classic oxygen regulated genes. Activation of the unfolded protein response (UPR) by tunicamycin, on the other hand, strongly induced ERO1-Lbeta and more moderately ERO1-Lalpha expression. The expression of the two ERO1-L isoforms therefore appears to be differently regulated, in the way that ERO1-Lalpha expression is mainly controlled by the cellular oxygen tension, whilst ERO1-Lbeta is triggered mainly by UPR. The physiological meaning of the oxygen regulation of ERO1-Lalpha expression likely is to maintain the transfer rate of oxidizing equivalents to PDI in situations of an altered cellular redox state induced by changes of the cellular oxygen tension.

An essential role of Sam50 in the protein sorting and assembly machinery of the mitochondrial outer membrane.

The preprotein translocase of the outer mitochondrial membrane (TOM complex) contains one essential subunit, the channel Tom40. The assembly pathway of the precursor of Tom40 involves the TOM complex and the sorting and assembly machinery (SAM complex) with the non-essential subunit Mas37. We have identified Sam50, the second essential protein of the mitochondrial outer membrane. Sam50 contains a beta-barrel domain conserved from bacteria to man and is a subunit of the SAM complex. Yeast mutants of Sam50 are defective in the assembly pathways of Tom40 and the abundant outer membrane protein porin, while the import of matrix proteins is not affected. Thus the protein sorting and assembly machinery of the mitochondrial outer membrane involves an essential, conserved protein.