Jolyon K. Claridge

Structural, Dynamic, and Chemical Characterization of a Novel S-Glycosylated Bacteriocin

Bacteriocins are bacterial peptides with specific activity against competing species. They hold great potential as natural preservatives and for their probiotic effects. We show here nuclear magnetic resonance-based evidence that glycocin F, a 43-amino acid bacteriocin from Lactobacillus plantarum, contains two β-linked N-acetylglucosamine moieties, attached via side chain linkages to a serine via oxygen, and to a cysteine via sulfur. The latter linkage is novel and has helped to establish a new type of post-translational modification, the S-linked sugar. The peptide conformation consists primarily of two α-helices held together by a pair of nested disulfide bonds. The serine-linked sugar is positioned on a short loop sequentially connecting the two helices, while the cysteine-linked sugar presents at the end of a long disordered C-terminal tail. The differing chemical and conformational stabilities of the two N-actetylglucosamine moieties provide clues about the possible mode of action of this bacteriostatic peptide.

A conserved amphipathic helix is required for membrane tubule formation by Yop1p

Significance The first structural studies, to our knowledge, of a reticulon homology domain (RHD), which is essential for maintaining smooth endoplasmic reticulum (ER) tubules and the edges of ER sheets, are described. We show here that the RHD of the protein Yop1p from the YOP1 gene has hydrophobic helices long enough to cross the membrane fully but contains a previously uncharacterized amphipathic helix (APH) that is necessary for membrane tubule formation. The APH is highly conserved in its amino acid properties and its location relative to the RHD both in the DP1 (deleted in polyposis) and reticulon families. These results place the DP1/reticulon proteins into the large and growing class of membrane-remodeling proteins that use APHs to influence membrane curvature. The integral membrane proteins of the DP1 (deleted in polyposis) and reticulon families are responsible for maintaining the high membrane curvature required for both smooth endoplasmic reticulum (ER) tubules and the edges of ER sheets, and mutations in these proteins lead to motor neuron diseases, such as hereditary spastic paraplegia. Reticulon/DP1 proteins contain reticulon homology domains (RHDs) that have unusually long hydrophobic segments and are proposed to adopt intramembrane helical hairpins that stabilize membrane curvature. We have characterized the secondary structure and dynamics of the DP1 family protein produced from the YOP1 gene (Yop1p) and identified a C-terminal conserved amphipathic helix (APH) that, on its own, interacts strongly with negatively charged membranes and is necessary for membrane tubule formation. Analyses of DP1 and reticulon family members indicate that most, if not all, contain C-terminal sequences capable of forming APHs. Together, these results indicate that APHs play a previously unrecognized role in RHD membrane curvature stabilization.

Fragments of Bacterial Endoglycosidase S and Immunoglobulin G Reveal Subdomains of Each That Contribute to Deglycosylation

Background: Endoglycosidase S (EndoS) is an immune evasion factor from Streptococcus pyogenes that impedes IgG effector functions by deglycosylation. Results: Analysis of fragments of enzyme and substrate identify components of each that contribute to catalysis. Conclusion: Regions outside of the catalytic domain of EndoS contribute to IgG deglycosylation, and catalysis does not require all antibody subunits. Significance: Engineering of EndoS specificity requires consideration of both catalytic and non-catalytic regions. Endoglycosidase S (EndoS) is a glycoside-hydrolase secreted by the bacterium Streptococcus pyogenes. EndoS preferentially hydrolyzes the N-linked glycans from the Fc region of IgG during infection. This hydrolysis impedes Fc functionality and contributes to the immune evasion strategy of S. pyogenes. Here, we investigate the mechanism of human serum IgG deactivation by EndoS. We expressed fragments of IgG1 and demonstrated that EndoS was catalytically active against all of them including the isolated CH2 domain of the Fc domain. Similarly, we sought to investigate which domains within EndoS could contribute to activity. Bioinformatics analysis of the domain organization of EndoS confirmed the previous predictions of a chitinase domain and leucine-rich repeat but also revealed a putative carbohydrate binding module (CBM) followed by a C-terminal region. Using expressed fragments of EndoS, circular dichroism of the isolated CBM, and a CBM-C-terminal region fusion revealed folded domains dominated by β sheet and α helical structure, respectively. Nuclear magnetic resonance analysis of the CBM with monosaccharides was suggestive of carbohydrate binding functionality. Functional analysis of truncations of EndoS revealed that, whereas the C-terminal of EndoS is dispensable for activity, its deletion impedes the hydrolysis of IgG glycans.

Solution Structure of the Squash Aspartic Acid Proteinase Inhibitor (SQAPI) and Mutational Analysis of Pepsin Inhibition

The squash aspartic acid proteinase inhibitor (SQAPI), a proteinaceous proteinase inhibitor from squash, is an effective inhibitor of a range of aspartic proteinases. Proteinaceous aspartic proteinase inhibitors are rare in nature. The only other example in plants probably evolved from a precursor serine proteinase inhibitor. Earlier work based on sequence homology modeling suggested SQAPI evolved from an ancestral cystatin. In this work, we determined the solution structure of SQAPI using NMR and show that SQAPI shares the same fold as a plant cystatin. The structure is characterized by a four-strand anti-parallel β-sheet gripping an α-helix in an analogous manner to fingers of a hand gripping a tennis racquet. Truncation and site-specific mutagenesis revealed that the unstructured N terminus and the loop connecting β-strands 1 and 2 are important for pepsin inhibition, but the loop connecting strands 3 and 4 is not. Using ambiguous restraints based on the mutagenesis results, SQAPI was then docked computationally to pepsin. The resulting model places the N-terminal strand of SQAPI in the S′ side of the substrate binding cleft, whereas the first SQAPI loop binds on the S side of the cleft. The backbone of SQAPI does not interact with the pepsin catalytic Asp32–Asp215 diad, thus avoiding cleavage. The data show that SQAPI does share homologous structural elements with cystatin and appears to retain a similar protease inhibitory mechanism despite its different target. This strongly supports our hypothesis that SQAPI evolved from an ancestral cystatin.

A picornaviral loop-to-loop replication complex

Abstract Picornaviruses replicate their RNA genomes through a highly conserved mechanism that involves an interaction between the principal viral protease (3Cpro) and the 5′-UTR region of the viral genome. The 3Cpro catalytic site is the target of numerous replication inhibitors. This paper describes the first structural model of a complex between a picornaviral 3Cpro and a region of the 5′-UTR, stem-loop D (SLD). Using human rhinovirus as a model system, we have combined NMR contact information, small-angle X-ray scattering (SAXS) data, and previous mutagenesis results to determine the shape, position and relative orientation of the 3Cpro and SLD components. The results clearly identify a 1:1 binding stoichiometry, with pronounced loops from each molecule providing the key binding determinants for the interaction. Binding between SLD and 3Cpro induces structural changes in the proteolytic active site that is positioned on the opposite side of the protease relative to the RNA/protein interface, suggesting that subtle conformational changes affecting catalytic activity are relayed through the protein.

Intrinsic disorder and coiled-coil formation in prostate apoptosis response factor 4

Prostate apoptosis response factor‐4 (Par‐4) is an ubiquitously expressed pro‐apoptotic and tumour suppressive protein that can both activate cell‐death mechanisms and inhibit pro‐survival factors. Par‐4 contains a highly conserved coiled‐coil region that serves as the primary recognition domain for a large number of binding partners. Par‐4 is also tightly regulated by the aforementioned binding partners and by post‐translational modifications. Biophysical data obtained in the present study indicate that Par‐4 primarily comprises an intrinsically disordered protein. Bioinformatic analysis of the highly conserved Par‐4 reveals low sequence complexity and enrichment in polar and charged amino acids. The high proteolytic susceptibility and an increased hydrodynamic radius are consistent with a largely extended structure in solution. Spectroscopic measurements using CD and NMR also reveal characteristic features of intrinsic disorder. Under physiological conditions, the data obtained show that Par‐4 self‐associates via the C‐terminal domain, forming a coiled‐coil. Interruption of self‐association by urea also resulted in loss of secondary structure. These results are consistent with the stabilization of the coiled‐coil motif through an intramolecular association.

Two‐state conformational equilibrium in the Par‐4 leucine zipper domain

Prostate apoptosis response factor‐4 (Par‐4) is a pro‐apoptotic and tumor‐suppressive protein. A highly conserved heptad repeat sequence at the Par‐4 C‐terminus suggests the presence of a leucine zipper (LZ). This C‐terminal region is essential for Par‐4 self‐association and interaction with various effector proteins. We have used nuclear magnetic resonance (NMR) spectroscopy to fully assign the chemical shift resonances of a peptide comprising the LZ domain of Par‐4 at neutral pH. Further, we have investigated the properties of the Par‐4 LZ domain and two point mutants under a variety of conditions using NMR, circular dichroism (CD), light scattering, and bioinformatics. Results indicate an environment‐dependent conformational equilibrium between a partially ordered monomer (POM) and a predominantly coiled coil dimer (CCD). The combination of techniques used allows the time scales of the equilibrium to be probed and also helps to identify features of the amino acid sequence that may influence the equilibrium. Proteins 2010.

Bacterial Production and Solution NMR Studies of a Viral Membrane Ion Channel

Advances in solution nuclear magnetic resonance (NMR) methodology that enable studies of very large proteins have also paved the way for studies of membrane proteins that behave like large proteins due to the added weight of surfactants. Solution NMR has been used to determine the high-resolution structures of several small, membrane proteins dissolved in detergent micelles and small bicelles. However, the usual difficulties with membrane proteins in producing, purifying, and stabilizing the proteins away from native membranes remain, requiring intensive screening efforts. Low levels of heterologous expression can be the most detrimental aspect to studying membrane proteins. This is exacerbated for NMR studies because of the costs of isotopically enriched media. Thus, solution NMR studies have tended to focus on relatively small, membrane proteins that can be expressed into inclusion bodies and refolded. Here, we describe the methods used to produce, purify, and refold the proton channel M2 into detergent micelles, and the procedures used to determine chemical shift assignments and the atomic level structure of the closed form of the homotetrameric channel.