We investigated the adsorption of bovine serum albumin (BSA) on colloidal Al2O3 particles in an aqueous environment. Changes in the zeta potential of the Al2O3 particles upon the adsorption of BSA were measured using an electro-acoustic technique. The mass of protein adsorbed was determined by using UV-vis spectroscopy. The change of the isoelectric point of the Al2O3 powder-protein suspension was found to be a function of adsorbed protein mass. It was shown that approximately one monolayer of BSA was needed to fully mask the surface and to compromise the charge of Al2O3. From titration experiments it follows that about 30-36% of the negatively charged groups of the protein form bonds with the protonated and charged Al2O3 surface. On the basis of our observations we introduced a new adsorption model for BSA on Al2O3 particles.

SummaryLipoproteins are a subgroup of secreted bacterial proteins characterized by a lipidated N-terminus, processing of which is mediated by the consecutive activity of prolipoprotein diacylglyceryl transferase (Lgt) and lipoprotein signal peptidase (LspA). The study of LspA function has been limited mainly to non-pathogenic microorganisms. To study a potential role for LspA in the pathogenesis of bacterial infections, we have disrupted lspA by allelic replacement in Mycobacterium tuberculosis , one of the world's most devastating pathogens. Despite the presence of an impermeable lipid outer layer, it was found that LspA was dispensable for growth under in vitro culture conditions. In contrast, the mutant was markedly attenuated in virulence models of tuberculosis. Our findings establish lipoprotein metabolism as a major virulence determinant of tuberculosis and define a role for lipoprotein processing in bacterial pathogenesis. In addition, these results hint at a promising new target for therapeutic intervention, as a highly specific inhibitor of bacterial lipoprotein signal peptidases is available.

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Lipoproteins are a functionally diverse class of secreted bacterial proteins characterized by an Nterminal lipid moiety. The lipid moiety serves to anchor these proteins to the cell surface. Lipoproteins are synthesized as pre-prolipoproteins and mature by post-translational modifications. The post-translational modifications are directed by the lipobox motif located within the signal peptide. Enzymes involved in lipoprotein synthesis are essential in Gram-negative bacteria but not in Gram-positve bacteria. Inactivation of genes involved in lipoprotein synthesis attenuates a variety of Gram-positive pathogens, including Mycobacterium tuberculosis. The attenuated phenotype of these mutants indicates an important role of lipoproteins and lipoprotein synthesis in bacterial virulence. M. tuberculosis, the causative agent of tuberculosis, is one of the most devastating pathogens in the world. This article reviews recent findings on the synthesis, localization and function of lipoproteins in mycobacteria. IntroductionLipoproteins are a functionally heterogeneous class of proteins universally present in bacteria; typically between 1 % and 3 % of bacterial genomes encode lipoproteins (Babu et al., 2006; http://www.mrc-lmb.cam.ac.uk/genomes/ dolop/). Lipoproteins represent a subgroup of secreted proteins characterized by the presence of a lipobox. The lipobox motif is located in the C-terminal part of the leader peptide and consists of four amino acids [LVI/ASTVI/GAS/ C] (Babu et al., 2006). This motif functions as a recognition signal for lipid modification, which is made on the conserved and essential cysteine residue. Precursor lipoproteins are mainly translocated in a Sec-dependent manner across the plasma membrane and are modified subsequently. Recent investigations indicate that lipoproteins may also be translocated by the twin-arginine translocation (Tat) system (McDonough et al., 2005). However, this pathway presumably is of minor importance because the number of lipoproteins translocated via the Tat transporter is rather small. Modification of precursor proteins is mediated by the consecutive activity of three enzymes: phosphatidylglycerol-pre-prolipoprotein diacylglyceryl transferase (Lgt), prolipoprotein signal peptidase/signal peptidase II (LspA) and phospholipid-apolipoprotein Nacyltransferase (Lnt) (Fig. 1). While Lgt and LspA are universally present in bacteria, Lnt has been reported to be restricted to Gram-negatives (Wu, 1996). The lipid residue covalently linked to the conserved cysteine moiety is thought to allow for anchoring of proteins in biological membranes by means of hydrophobic interaction. In Grampositive bacteria, cell-associated lipoproteins are found in the plasma membrane. In Gram-negative bacteria the majority of cell-associated lipoproteins are found in the outer membrane -only about 10 % stay anchored in the plasma membrane. Lipoproteins may be divided into five general groups according to their function in adhesion and invasion, cell wall synthesis, nutrient uptake, degradative process...

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