微生物学

Legionella pneumophila, a Gram-negative bacterium and the causative agent of Legionnaires’ disease, exports over 300 effector proteins/virulence factors, through its type II (T2SS) and type IV secretion systems (T4SS). One such T2SS virulence factor, ChiA, not only functions as a chitinase, but also as a novel mucinase, which we believe aids ChiA-dependent virulence during lung infection. Previously published protocols manipulated wild-type L. pneumophila strain 130b and its chiA mutant to express plasmid-encoded GFP. Similarly, earlier studies demonstrated that wheat germ agglutinin (WGA) can be fluorescently labeled and can bind to mucins. In the current protocol, GFP-labeled bacteria were incubated with type II and type III porcine stomach mucins, which were then labeled with TexasRed-tagged WGA and analyzed by flow-cytometry to measure the binding of bacteria to mucins in the presence or absence of endogenous ChiA. In addition, we analysed binding of purified ChiA to type II and type III porcine stomach mucins. This protocol couples both bacterial and direct protein binding to mucins and is the first to measure Gram-negative bacterial binding to mucins using WGA and flow-cytometric analysis.

Graphic abstract:

Strategy for assessing bacterial and protein binding to mucins

DNA footprinting is a classic technique to investigate protein-DNA interactions. However, traditional footprinting protocols can be unsuccessful or difficult to interpret if the binding of the protein to the DNA is weak, the protein has a fast off-rate, or if several different protein-DNA complexes are formed. Our protocol differs from traditional footprinting protocols, because it provides a method to isolate the protein-DNA complex from a native gel after treatment with the footprinting agent, thus removing the bound DNA from the free DNA or other protein-DNA complexes. The DNA is then extracted from the isolated complex before electrophoresis on a sequencing gel to determine the footprinting pattern. This analysis provides a possible solution for those who have been unable to use traditional footprinting methods to determine protein-DNA contacts.

Histone post-translational modifications (PTMs) regulate numerous cellular processes, including gene transcription, cell division, and DNA damage repair. Most histone PTMs affect the recruitment or exclusion of reader proteins from chromatin. Here, we present a protocol to measure affinity and interaction kinetics between histone peptides and the recombinant protein using Bio-layer interferometry.

Nucleosomes are the fundamental unit of eukaryotic chromosome packaging, comprised of 147 bp of DNA wrapped around two molecules of each of the core histone proteins H2A, H2B, H3, and H4. Nucleosomes are symmetrical, with one axis of symmetry centered on the homodimeric interaction between the C-termini of the H3 molecules. To explore the functional consequences of nucleosome symmetry, we designed an obligate pair of H3 heterodimers, termed H3X and H3Y, allowing us to compare cells with single or double H3 alterations. Our biochemical validation of the heterodimeric X-Y interaction included intra-nucleosomal H3 crosslinking using dimethyl suberimidate (DMS). Here, we provide a detailed protocol for the use of DMS to analyze yeast nucleosomes.

This protocol describes the production of GST-Cbx7 fusion proteins from E. coli, originally developed in the recent publication (Zhen et al., 2016). The pGEX-6P-1-GST plasmids encoding the Cbx7 variants were transformed into BL21 competent cells. The fusion protein production was induced by isopropyl-beta-D-thiogalactopyranoside and they were purified by Glutathione Sepharose 4B. This protocol can be adapted for the purification of other proteins.

The ability to stabilize other proteins against thermal aggregation is one of the major characteristics of chaperone proteins. Molecular chaperones bind to nonnative conformations of proteins. Folding of the substrate is triggered by a dynamic association and dissociation cycles which keep the substrate protein on track of the folding pathway (Figure 1). Usually molecular chaperones exhibit differential affinities with different conformations of the substrate. With the exception of the sHsp family of molecular chaperones, the shift from a high-affinity binding state to the low-affinity release state is triggered by ATP binding and hydrolysis (Haselback and Buchner, 2015). Aggregation prevention assay is a simple, yet definitive assay to determine the chaperone activity of heat labile proteins such as Maltodextrin glucosidase (MalZ), Citrate Synthase (CS) and NdeI. This is based on the premise that proteins with chaperone like activity should prevent protein substrates (MalZ, CS and NdeI) from thermal aggregation. Here, we describe a detailed protocol for aggregation prevention assay using two different chaperone proteins, resistin and MoxR1, identified from our lab. Resistin, a human protein (hRes) and MoxR1 a Mycobacterium tuberculosis protein were analysed for their effect on prevention of MalZ/Citrate Synthase (CS)/NdeI aggregation.


Figure 1. Mechanism of action of molecular chaperones. Citrate synthase folds via increasingly structured intermediates (I₁, I₂) from the unfolded state (U) to the folded state (N). Under heat shock conditions, this process is reversed.

Heavy metals can cause damage to biomolecules such as proteins and DNA in multiple ways. Cells therefore strive for keeping intracellular (heavy) metal ions bound to specific proteins that are capable of handling detoxification, export or integration as cofactors. Metal binding proteins usually provide specific coordination sites that bind certain ions with ultrahigh affinity, with the thermodynamic driving force being the stability of organometallic complexes. However, the metal binding properties of these proteins can be highly variable. Therefore the transfer of specific ions between separate proteins or even between distinct binding sites located on one and the same protein does not always follow affinity gradients, but depends on particular protein interactions that are difficult to predict. We established a method suitable to probe metal transfer between two proteins, provided the proteins are amenable to purification and in vitro handling. It consists of the loading with metals, the co-incubation and the separation of metal-exchanging proteins with subsequent determination of bound metal content. The method is exemplified by experimental data of ours probing the transfer of copper(I) between the membrane-extrinsic metal binding domain MBD2 and the transmembrane domain of CopA, a copper export ATPase from Escherichia coli (Drees et al., 2015).

The displacement assay was designed to quantify the direct competition between two homologous ribosomal proteins from Mycobacterium tuberculosis, S18-1 and S18-2, for interaction with their cognate binding partner, ribosomal protein S6 (Prisic et al., 2015). The S18 proteins were dialyzed in two physiologically relevant conditions (i.e. in the presence of Zn²⁺ or with EDTA to chelate Zn²⁺) and then allowed to compete for binding to S6 which was maintained in limiting concentration. The result was obtained through an ELISA, where S6-His is first bound to a Ni²⁺-NTA plate, followed by addition of S18-2 in excess to S6, then by addition of increasing concentrations of S18-1. The percentage of S18-2 that remained bound to S6 was quantified with antibodies specific to the S18-2 protein and secondary antibodies, in chemiluminescent ELISA. In this way displacement of S18-2 protein by the S18-1 protein was reported as a percentage of the full strength signal achieved through saturation of S6 with S18-2. At its foundation, this method exploits a native protein-protein interaction and could be applied to other systems where two or more proteins compete for binding to a target ligand as above.

Chromatin immunoprecipitation (ChIP) is a powerful technology for analyzing protein-DNA interactions in cells. Robust ChIP procedures have been established for investigating direct interactions between protein and DNA. However, detecting indirect protein-DNA interactions in vivo is challenging. Recently, we used ChIP to analyze an indirect protein-DNA interaction between a putative histone demethylase, MoJmjC, and the promoter of the superoxide dismutase 1-encoding gene MoSOD1 in the rice blast fungus Magnaporthe oryzae (M. oryzae) (Fernandez et al., 2014). We tagged MoJmjC with the 3x FLAG epitope (Fernandez et al., 2014), instead of the larger and more commonly used GFP epitope, to mitigate against steric hindrance. We also employed a two-step cross-linking strategy using DSG and formaldehyde-rather than the one-step formaldehyde cross-linking procedure more frequently employed for analyzing direct protein-DNA interactions - in order to better capture the indirect MoJmjC-MoSOD1 DNA interactions in vivo. In addition, we have shown that two-step cross-linking is suitable for ChIP analysis of direct protein-DNA interactions between a GATA transcription factor, Asd4, and its cognate binding site (Marroquin-Guzman and Wilson, 2015). Here, we provide a detailed protocol for chromatin immunoprecipitation, with versatile two-step cross-linking, in M. oryzae.

Kinetic analysis of antibodies is one of the important studies for characterization of antibodies and screening of ligands. In our recent study (Ingale et al., 2014), we compared the antigenic profiles and binding characteristics of four HIV-1 envelope glycoprotein (Env) core immunogens using multiple monoclonal antibodies by Bio-Layer Light Interferometry (BLI). This technology enables real-time analysis of interactions on the surface of a fiber optic biosensor by accurately measuring kinetic constants such as K_a, K_d, and K_D in a 96-well format.