WAFL, a new protein involved in regulation of early endocytic transport at the intersection of actin and microtubule dynamics.

We have previously identified a new gene with sequence homology to the WASP-family of actin regulators denoted WAFL (WASP and FKBP-like). Here we report a possible biological function for WAFL, by demonstrating an association to early endosomes via its central coiled-coil domain. Further we show by functional and structural studies that WAFL is associated with both microtubules and the actin filament system, the two means of transport of early endosomes. In addition, WAFL interacts with WASP-interacting protein (WIP) and actin, thus linking WAFL to actin dynamics. The use of RNAi depletion of WAFL shows that WAFL-deficient cells display delayed transport of endosomal cargo. Our findings are compatible with a model whereby WAFL is involved in the transport of early endosomes at the level of transition between microfilament-based and microtubule-based movement.

The C-terminal of CysM from Mycobacterium tuberculosis protects the aminoacrylate intermediate and is involved in sulfur donor selectivity.

A new crystal structure of the dimeric cysteine synthase CysM from Mycobacterium tuberculosis reveals an open and a closed conformation of the enzyme. In the closed conformation the five carboxy-terminal amino acid residues are inserted into the active site cleft. Removal of this segment results in a decreased lifetime of the alpha-aminoacrylate reaction intermediate, an increased sensitivity to oxidants such as hydrogen peroxide, and loss of substrate selectivity with respect to the sulfur carrier thiocarboxylated CysO. These results highlight features of CysM that might be of particular importance for cysteine biosynthesis under oxidative stress in M. tuberculosis.

1.9 A structure of the signal receiver domain of the putative response regulator NarL from Mycobacterium tuberculosis.

NarL from Mycobacterium tuberculosis is a putative nitrate response regulator that is involved in the regulation of anaerobic metabolism in this pathogen. The recombinant purified N-terminal signal receiver domain of NarL has been crystallized in space group C222(1), with unit-cell parameters a = 85.6, b = 90.0, c = 126.3 A, and the structure was determined by molecular replacement to 1.9 A resolution. Comparisons with related signal receiver domains show that the closest structural homologue is an uncharacterized protein from Staphylococcus aureus, whereas the nearest sequence homologue, NarL from Escherichia coli, displays larger differences in three-dimensional structure. The largest differences between the mycobacterial and E. coli NarL domains were found in the loop between beta3 and alpha3 in the proximity of the phosphorylation site. The active site in response regulators is similar to that of members of the haloacid dehalogenase (HAD) family, which also form a phospho-aspartyl intermediate. In NarL, the aspartic acid that acts as catalytic acid/base in several HAD enzymes is replaced by an arginine residue, which is less likely to participate in steps involving proton abstraction. This substitution may slow down the breakdown of the phospho-aspartyl anhydride and allow signalling beyond the timescales defined by a catalytic reaction intermediate.

Cysteine synthase (CysM) of Mycobacterium tuberculosis is an O-phosphoserine sulfhydrylase: evidence for an alternative cysteine biosynthesis pathway in mycobacteria.

The biosynthesis of cysteine is a crucial metabolic pathway supplying a building block for de novo protein synthesis but also a reduced thiol as a component of the oxidative defense mechanisms that appear particularly vital in the dormant state of Mycobacterium tuberculosis. We here show that the cysteine synthase CysM is, in contrast to previous annotations, an O-phosphoserine-specific cysteine synthase. CysM belongs to the fold type II pyridoxal 5'-phosphate-dependent enzymes, as revealed by the crystal structure determined at 2.1-angstroms resolution. A model of O-phosphoserine bound to the enzyme suggests a hydrogen bonding interaction of the side chain of Arg220 with the phosphate group as a key feature in substrate selectivity. Replacement of this residue results in a significant loss of specificity for O-phosphoserine. Notably, reactions with sulfur donors are not affected by the amino acid replacement. The specificity of CysM toward O-phosphoserine together with the previously established novel mode of sulfur delivery via thiocarboxylated CysO (Burns, K. E., Baumgart, S., Dorrestein, P. C., Zhai, H., McLafferty, F. W., and Begley, T. P. (2005) J. Am. Chem. Soc. 127, 11602-11603) provide strong evidence for an O-phosphoserine-based cysteine biosynthesis pathway in M. tuberculosis that is independent of both O-acetylserine and the sulfate reduction pathway. The existence of an alternative biosynthetic pathway to cysteine in this pathogen has implications for the design strategy aimed at inhibition of this metabolic route.

Three-dimensional structures of apo- and holo-L-alanine dehydrogenase from Mycobacterium tuberculosis reveal conformational changes upon coenzyme binding.

L-alanine dehydrogenase from Mycobacterium tuberculosis catalyzes the NADH-dependent reversible conversion of pyruvate and ammonia to L-alanine. Expression of the gene coding for this enzyme is up-regulated in the persistent phase of the organism, and alanine dehydrogenase is therefore a potential target for pathogen control by antibacterial compounds. We have determined the crystal structures of the apo- and holo-forms of the enzyme to 2.3 and 2.0 A resolution, respectively. The enzyme forms a hexamer of identical subunits, with the NAD-binding domains building up the core of the molecule and the substrate-binding domains located at the apical positions of the hexamer. Coenzyme binding stabilizes a closed conformation where the substrate-binding domains are rotated by about 16 degrees toward the dinucleotide-binding domains, compared to the open structure of the apo-enzyme. In the structure of the abortive ternary complex with NAD+ and pyruvate, the substrates are suitably positioned for hydride transfer between the nicotinamide ring and the C2 carbon atom of the substrate. The approach of the nucleophiles water and ammonia to pyruvate or the reaction intermediate iminopyruvate, respectively, is, however, only possible through conformational changes that make the substrate binding site more accessible. The crystal structures identified the conserved active-site residues His96 and Asp270 as potential acid/base catalysts in the reaction. Amino acid replacements of these residues by site-directed mutagenesis led to inactive mutants, further emphasizing their essential roles in the enzymatic reaction mechanism.

Arrays of vertical-cavity electroabsorption modulators for parallel signal processing.

Vertical-cavity multiple quantum well electroabsorption modulators (EAM) offer gigahertz modulation speeds, insensitivity to light polarisation and can be integrated into large arrays. They are therefore good candidates for efficient parallel signal processing architectures. We present high-performance 2 x 128 and 2 x 64 EAM arrays that were fabricated at 4'' wafer-scale by using optimised fabrication and hybridisation processes. The arrays exhibit contrast ratios of 20:1 with a voltage swing of 1 V, a maximum contrast ratio of 335:1 for a 10 V bias and a modulation frequency in excess of 15 MHz.