Despite the great progress recently made in resolving their structures, investigation of the structural biology of membrane proteins still presents major challenges. Even with new technical advances such as lipidic cubic phase crystallisation, obtaining well-ordered crystals remains a significant hurdle in membrane protein X-ray crystallographic studies. As an alternative, electron microscopy has been shown to be capable of resolving > 3.5 Å resolution detail in membrane proteins of modest (~ 300 kDa) size, without the need for crystals. However, the conventional use of detergents for either approach presents several issues, including the possible effects on structure of removing the proteins from their natural membrane environment. As an alternative, it has recently been demonstrated that membrane proteins can be effectively isolated, in the absence of detergents, using a styrene maleic acid co-polymer (SMA). This approach yields SMA lipid particles (SMALPs) in which the membrane proteins are surrounded by a small disk of lipid bilayer encircled by polymer. Here we use the Escherichia coli secondary transporter AcrB as a model membrane protein to demonstrate how a SMALP scaffold can be used to visualise membrane proteins, embedded in a near-native lipid environment, by negative stain electron microscopy, yielding structures at a modest resolution in a short (days) timeframe. Moreover, we show that AcrB within a SMALP scaffold is significantly more active than the equivalent DDM stabilised form. The advantages of SMALP scaffolds within electron microscopy are discussed and we conclude that they may prove to be an important tool in studying membrane protein structure and function.

Oxidation of methionine residues to methionine sulfoxide can lead to inactivation of proteins. Methionine sulfoxide reductase (MsrA) has been known for a long time, and its repairing function well characterized. Here we identify a new methionine sulfoxide reductase, which we referred to as MsrB, the gene of which is present in genomes of eubacteria, archaebacteria, and eucaryotes. The msrA and msrB genes exhibit no sequence similarity and, in some genomes, are fused. The Escherichia coli MsrB protein (currently predicted to be encoded by an open reading frame of unknown function named yeaA) was used for genetic, enzymatic, and mass spectrometric investigations. Our in vivo study revealed that msrB is required for cadmium resistance of E. coli, a carcinogenic compound that induces oxidative stress. Our in vitro studies, showed that (i) MsrB and MsrA enzymes reduce free methionine sulfoxide with turnover rates of 0.6 min ؊1 and 20 min ؊1 , respectively, (ii) MsrA and MsrB act on oxidized calmodulin, each by repairing four to six of the eight methionine sulfoxide residues initially present, and (iii) simultaneous action of both MsrA and MsrB allowed full reduction of oxidized calmodulin. A possibility is that these two ubiquitous methionine sulfoxide reductases exhibit different substrate specificity.

Today, proteomics is an exciting approach to discover potential biomarkers of different disorders. One challenge with proteomics experiments is the wide concentration range of proteins in various tissues and body fluids. The most abundant component in human body fluids, human serum albumin (HSA), is present at concentrations corresponding to approximately 50% of the total protein content in, e.g., plasma and cerebrospinal fluid (CSF). If this component could be selectively removed, then the chances of observing lower-abundance component of clinical interest would be greatly improved. There are today several approaches of varying specificity available for depletion. In this study, the properties of two commercially available kits, for the removal of HSA and HSA and immunoglobulin G (IgG), respectively, were compared, and the benefits of using depletion steps prior to on-line LC-FTICR MS were evaluated. Both methods were applied on plasma and CSF. To our knowledge, these are the first results reported for CSF. Also, the combination with electrospray LC-FTICR MS is novel. The proportion of depleted HSA and IgG was estimated using global labeling markers for peptide quantification. Both depletion-methods provided a significant reduction of HSA, and the identification of lower abundant components was clearly facilitated. A higher proportion of HSA was removed using the affinity-based removal kit, and consequently more proteins could be identified using this approach.

Chemical investigations of a microfungus Xylaria sp. isolated from the Australian rainforest tree Glochidion ferdinandi have afforded two new natural products, 2-hydroxy-6-methyl-8-methoxy-9-oxo-9H-xanthene-1-carboxylic acid (1) and 2-hydroxy-6-hydroxymethyl-8-methoxy-9-oxo-9H-xanthene-1-carboxylic acid (2). Compound 1 has previously been synthesised but only partially characterised. Methylation of 1 using diazomethane afforded the crystalline compound 2,8-dimethoxy-6-methyl-9-oxo-9H-xanthene-1-carboxylic acid methyl ester (3), whose structure was determined by single crystal X-ray analysis. This paper reports the full spectroscopic characterisation of compounds 1-3 by NMR, UV, IR and MS data. All compounds were inactive in a brine shrimp lethality assay and several antimicrobial screens.

No abstract