Structure-function analysis of human stomatin: A mutation study.

Stomatin is an ancient, widely expressed, oligomeric, monotopic membrane protein that is associated with cholesterol-rich membranes/lipid rafts. It is part of the SPFH superfamily including stomatin-like proteins, prohibitins, flotillin/reggie proteins, bacterial HflK/C proteins and erlins. Biochemical features such as palmitoylation, oligomerization, and hydrophobic "hairpin" structure show similarity to caveolins and other integral scaffolding proteins. Recent structure analyses of the conserved PHB/SPFH domain revealed amino acid residues and subdomains that appear essential for the structure and function of stomatin. To test the significance of these residues and domains, we exchanged or deleted them, expressed respective GFP-tagged mutants, and studied their subcellular localization, molecular dynamics and biochemical properties. We show that stomatin is a cholesterol binding protein and that at least two domains are important for the association with cholesterol-rich membranes. The conserved, prominent coiled-coil domain is necessary for oligomerization, while association with cholesterol-rich membranes is also involved in oligomer formation. FRAP analyses indicate that the C-terminus is the dominant entity for lateral mobility and binding site for the cortical actin cytoskeleton.

Identification and validation of platelet low biological variation proteins, superior to GAPDH, actin and tubulin, as tools in clinical proteomics.

Accurate biomarker quantification requires carefully chosen normalisation procedures. When single proteins are used as loading controls (LCs), it is crucial that their expressional stability must be known. Platelets are an important biomarker source, especially for neurological diseases. We performed a systematical analysis of the platelet proteome to identify proteins suitable as LCs, using the 2-D DIGE system. We first screened a healthy population (n=137), aged between 18 and 104years, to find proteins with small coefficients of total variation (CVtot), herein termed low biological variation proteins (LBVP). Thereafter, expressional stability was verified in 101 patients suffering from Alzheimer's- (AD), Parkinson's- disease, vascular dementia or schizophrenia. Interestingly, traditional LCs such as tubulin beta-1 and GAPDH, were not found amongst LBVP. The least variable protein, calculated over all 238 individuals, was 14-3-3 gamma, with a CVtot of 9.3%, showing no gender, age or disease dependency. The normalisation capability of 14-3-3 gamma was superior to traditional LC in quantifying Western blot signals of the platelet AD-biomarker Monoamine Oxidase B of patient versus controls. Similar results were obtained with HepG2 cells, treated in vitro with DNA-methyltransferase inhibitor 5-aza-2'deoxicytidine. Finally, we provide a list of alternative normalisation candidates for accurate biomarker quantification. BIOLOGICAL SIGNIFICANCE: This paper suggests a considerable list of platelet proteins with a lower biological variation than well known "housekeeping" proteins like GAPDH and tubulin. Spot abundances of found proteins are middle ranged and unaffected by gender, age and certain diseases. Hence, listed proteins might be valuable normalisation candidates used additionally or alternatively. Platelet's least variable protein 14-3-3 gamma is validated as normalisation protein in platelet biomarker quantification. Furthermore 14-3-3 gamma is demonstrated to be also stable expressed by in HepG2, cells others than platelets, when treated by DNA methylation inhibitor.

The role of proteomics in dementia and Alzheimer's disease.

Proteomic analysis enables us to identify dementia-related protein profiles of both genetic and environmental origins. In this review, current proteomics technologies are described including many examples of clinical proteomics studies. Many of these studies present only results of the discovery phase. Progression to the validation phase was achieved by developing more advanced proteomics technologies such as fluorescence two-dimensional differential gel electrophoresis or isobaric tagging for relative and absolute protein quantification. These technologies will lead to the design of several new Alzheimer's disease-related protein panels for the analysis of CSF. On these new panels, established markers such as tau and Abeta42 will be used in combination with novel markers, for example beta-2-microglobulin, brain-derived neurotrophic factor 1 and fragments of cystatin C. However, there are still limitations to using proteomic assays. The preparation of homogeneous sample material is difficult due the complexity of brain tissue. Laser capture microdissection and recently developed more sensitive proteomics methods, for example fluorescence saturation labelling, will overcome these limitations. Combining proteomics with approaches at the level of the genome and transcriptome followed by interpretation by systems biology will soon shed further light on dementia-related pathogenesis.

Characterization of the stomatin domain involved in homo-oligomerization and lipid raft association.

The cytoplasmically oriented monotopic integral membrane protein stomatin forms high-order oligomers and associates with lipid rafts. To characterize the domains that are involved in oligomerization and detergent-resistant membrane (DRM) association, we expressed truncation and point mutants of stomatin and analyzed their size and buoyancy by ultracentrifugation methods. A small C-terminal region of stomatin that is largely hydrophobic, Ser-Thr-Ile-Val-Phe-Pro-Leu-Pro-Ile (residues 264-272), proved to be crucial for oligomerization, whereas the N-terminal domain (residues 1-20) and the last 12 C-terminal amino acids (residues 276-287) were not essential. The introduction of alanine substitutions in the region 264-272 resulted in the appearance of monomers. Remarkably, only three of these residues, Ile-Val-Phe (residues 266-268), were found to be indispensable for the DRM association. Interestingly, the exchange of Pro-269 and to some extent the residues 270-272, which are essential for oligomerization, did not affect the DRM association of stomatin. This suggests that the formation of oligomers is not necessary for the association of stomatin with DRMs. Internal deletions near the membrane anchoring domain resulted in the formation of intermediate size oligomers suggesting a conformational interdependence of large parts of the C-terminal region. Fluorescence recovery after photobleaching analysis of the tagged, monomeric, non-DRM mutant ST-(1-262)-green fluorescent protein and wild type stomatin StomGFP showed a significantly higher lateral mobility of the truncation mutant in the plasma membrane suggesting a membrane interaction of the respective C-terminal region also in vivo.