微生物学

Glutamylation is a posttranslational modification where the amino group of a free glutamate amino acid is conjugated to the carboxyl group of a glutamate side chain within a target protein. SidJ is a Legionella kinase-like protein that has recently been identified to perform protein polyglutamylation of the Legionella SdeA Phosphoribosyl-Ubiquitin (PR-Ub) ligase to inhibit SdeA’s activity. The attachment of multiple glutamate amino acids to the catalytic glutamate residue of SdeA by SidJ inhibits SdeA’s modification of ubiquitin (Ub) and ligation activity. In this protocol, we will discuss a SidJ non-radioactive, in vitro glutamylation assay using its substrate SdeA. This will also include a second reaction to assay the inhibition of SdeA by using both modification of free Ub and ligation of ADP-ribosylated Ubiquitin (ADPR-Ub) to SdeA’s substrate Rab33b. Prior to the identification and publication of SdeA’s activity, no SdeA inhibition assays existed. Our group and others have demonstrated various methods to display inhibition of SdeA’s activity. The alternatives include measurement of ADP-ribosylation of Ub using radioactive NAD, NAD hydrolysis, and Western blot analysis of HA-Ub ligation by SdeA. This protocol will describe the inhibition of both ubiquitin modification and the PR-Ub ligation by SdeA using inexpensive standard gels and Coomassie staining.

The Legionella effector protein SidJ has recently been identified to perform polyglutamylation on another Legionella effector, SdeA, ablating SdeA’s activity. SidJ is a kinase-like protein that requires the small eukaryotic protein calmodulin to perform glutamylation. Glutamylation is a relatively uncommon type of post-translational modification, where the amino group of a free glutamate amino acid is covalently linked to the γ-carboxyl group of a glutamate sidechain in a substrate protein. This protocol describes the SidJ glutamylation reaction using radioactive [U-¹⁴C] glutamate and its substrate SdeA, the separation of proteins by gel electrophoresis, preparation of gels for radioactive exposure, and relative quantification of glutamylation activity. This procedure is useful for the identification of substrates for glutamylation, characterization of substrate and glutamylase activities due to mutations, and identification of proteins with glutamylation activity. Some studies have assayed glutamylation with the use of [³H] glutamate (Regnard et al., 1998) and the use of the GT335 antibody (Wolff et al., 1992). However, the use of [U-¹⁴C] glutamate requires a shorter radioactive exposure time with no dependence on antibody specificity.

Protein acetylation is one of the standard post-translational modifications found in proteins across all organisms, along with phosphorylation which regulates diverse cellular processes. Acetylation of proteins can be enzymatically catalyzed through acetyltransferases, acetyl CoA synthetases or non-enzymatically through acyl carrier metabolic intermediates. In this protocol, using response regulator proteins as targets we describe the experimental strategy for probing the occurrence of acetylation using purified recombinant proteins in an in vitro setup. Further using M. smegmatis strains overexpressing the wild type or mutant response regulator protein, we also describe how in vivo acetylation can be validated in Mycobacterial proteins. The described approach can be used for analyzing acetylation of any mycobacterial protein under both in vitro and in vivo conditions.

We recently investigated the molecular events that drive evolution of the CTX-M-type β-lactamases by DNA shuffling of fragments of the bla_CTX-M-14 and bla_CTX-M-15 genes. Analysis of a total of 51 hybrid enzymes showed that enzymatic activity could be maintained in most cases, yet the enzymatically active hybrids were found to possess much fewer amino acid substitutions than the few hybrids that became inactive, suggesting that point mutations in the constructs rather than reshuffling of the fragments of the two target genes would more likely cause disruption of CTX-M activity. Certain important residues that played important functional roles in mediating enzyme activity were identified. These findings suggest that DNA shuffling is an effective approach to identify and characterize important functional domains in bacterial proteins.

The hepatitis B virus (HBV) is an important global human pathogen and represents a major cause of hepatitis, liver cirrhosis and liver cancer. The HBV capsid is composed of multiple copies of a single viral protein, the capsid or core protein (HBc), plays multiple roles in the viral life cycle, and has emerged recently as a major target for developing antiviral therapies against HBV infection. Although several systems have been developed to study HBV capsid assembly, including heterologous overexpression systems like bacteria and insect cells, in vitro assembly using purified protein, and mammalian cell culture systems, the requirement for non-physiological concentrations of HBc and salts and the difficulty in manipulating host regulators of assembly presents major limitations for detailed studies on capsid assembly under physiologically relevant conditions. We have recently developed a mammalian cell-free system based on the rabbit reticulocyte lysate (RRL), in which HBc is expressed at physiological concentrations and assembles into capsids under near-physiological conditions. This system has already revealed HBc assembly requirements that are not anticipated based on previous assembly systems. Furthermore, capsid assembly in this system is regulated by endogenous host factors that can be readily manipulated. Here we present a detailed protocol for this cell-free capsid assembly system, including an illustration on how to manipulate host factors that regulate assembly.

The Actin-Related Protein 2/3 (ARP2/3) complex is an actin nucleator that generates a branched actin network in mammalian cells. In addition to binding nucleation promoting factors, LeClaire et al. demonstrated that its phosphorylation state is essential key for its activity (LeClaire et al., 2008). In cells, the ARP2/3 complex is phosphorylated on threonine and tyrosine residues of the ARP2, ARP3, and ARPC1 subunits (Vadlamudi et al., 2004; LeClaire et al., 2008; Narayanan et al., 2011; LeClaire et al., 2015). In particular, phosphorylation of threonine 237 and 238 of the ARP2 subunit is necessary to allow a change in the ARP2/3 complex structure to its active conformation (Narayanan et al., 2011; LeClaire et al., 2015). While important for many functions in eukaryotic cells, ARP2/3 complex activity also benefits several cellular pathogens (Haglund and Welch, 2011; Welch and Way, 2013). Recently, we demonstrated that the bacterial pathogen, Legionella pneumophila, manipulates ARP2/3 complex phosphorylation state using a bacterial protein kinase injected in host cell cytoplasm (Michard et al., 2015). Here, we describe how to test the ability of a bacterial protein kinase or another protein kinase to phosphorylate the ARP2/3 complex in an in vitro context. First, the ARP2/3 complex and the bacterial protein kinase are produced and purified. Then, the purified proteins are incubated in the presence of ATP, and the ARP2/3 phosphorylation level is analyzed by Western blot.

Inorganic polyphosphate (polyP) is a linear polymer present in both prokaryotic and eukaryotic organisms and made from three to hundreds of orthophosphate residues linked by phosphoanhydride bonds. The biological role of this molecule goes beyond serving as Pi store or energy source to replace ATP. For instance, in yeast polyP levels have been related to stress adaptation and this molecule has been shown to be the substrate for polyphosphorylation of proteins. Here we describe two different methods to purify polyP from the yeast Saccharomyces cerevisiae and the subsequent protocol to quantify polyP levels by spectrophotometrically measuring the Pi generated upon enzymatic hydrolysis of purified polyP. It must be noted that the purification protocol used greatly influences the polyP values obtained.


Figure 1. Enzymatic hydrolysis of polyP

Nedd8 is a small ubiquitin-like protein (9 kDa) covalently attached to a conserved lysine residue of a cullin protein which is part of cullin-RING ligases (CRLs). CRLs are major E3 ligases important for protein ubiquitination in the ubiquitin-proteasome pathway (UPP). The activity of CRLs is regulated by cycles of neddylation (CulA-N8, ~98 kDa) and deneddylation (CulA ~89 kDa). The COP9 signalosome (CSN) and Deneddylase A (DenA) are capable of cleaving the isopeptide bond between Nedd8 and CullinA. In contrast to the single protein DenA, CSN is an eight subunit multiprotein complex. Protein crude extracts of different Aspergillus nidulans csn deletion strains were mixed with recombinant CSN subunits expressed and purified from Escherichia coli (E. coli). Western hybridization experiments using anti-CulA or anti-Nedd8 antibodies could show the ratio of neddylated vs. deneddylated CulA. Using the deneddylation assay, we could show that CsnE is the last subunit joining a 7-subunit pre-assembled CSN in vitro and only then CSN can perform cullin deneddylation by the metalloprotease subunit CsnE. This assay is a fast and non-expensive method, which visualizes enzyme activity for deneddylating proteins. It might be also useful for testing the activity of other isopeptidases removing post-translational modifications from substrates in Aspergillus nidulans (A. nidulans) or other organisms.

Among the seven serines and one threonine in the carboxyl-terminus of HBV C protein, all but one (serine 183) appear in the context of RxxS/T consensus phosphoacceptor motifs and also overlap with other consensus motifs, such as S/TP, RS, SPRRR, RRRS/T, or RRxS/T, suggesting that various cellular kinases phosphorylate these residues. To determine whether threonine and/or serine (serines 157, 164, 170, 172, 178, and 180, and threonine 162, adw subtype) of HBV C protein are indeed phosphoacceptor residues in cells, Huh7 were transfected with a series of C-protein-expressing mutants, labeled with ³²P-orthophosphate for 14 h, and then lysed. The ³²Pi-labeled lysates were immunoprecipitated with anti-HBc antibody, and the ³²Pi-labeled immunoprecipitated C proteins were detected by autoradiography.

Interactions between pathogenic bacteria and host cells are often mediated by proteins found on the surfaces of the bacteria. The Gram-negative bacterium Helicobacter pylori is predicted to produce at least 50 surface-exposed outer membrane proteins, but there has been relatively little progress in experimentally analyzing the cell-surface proteome of this organism. Herein, we describe in detail a protocol that allows biotinylation and purification of surface-exposed H. pylori proteins. A comparative analysis of surface-exposed proteins identified by this biotinylation-based approach and by several other independent methods is described in a recent publication (Voss et al., 2014).