Characterization of the cofactor-induced folding mechanism of a zinc-binding peptide using computationally designed mutants

Non-native fold of the putative VPS39 zinc finger domain

Background: The multi-subunit homotypic fusion and vacuole protein sorting (HOPS) membrane-tethering complex is involved in regulating the fusion of late endosomes and autophagosomes with lysosomes in eukaryotes. The C-terminal regions of several HOPS components have been shown to be required for correct complex assembly, including the C-terminal really interesting new gene (RING) zinc finger domains of HOPS components VPS18 and VPS41. We sought to structurally characterise the putative C-terminal zinc finger domain of VPS39, which we hypothesised may be important for binding of VPS39 to cellular partners or to other HOPS components.

Methods: We recombinantly expressed, purified and solved the crystal structure of the proposed zinc-binding region of VPS39.

Results: In the structure, this region forms an anti-parallel β-hairpin that is incorporated into a homotetrameric eight-stranded β-barrel. However, the fold is stabilised by coordination of zinc ions by residues from the purification tag and an intramolecular disulphide bond between two predicted zinc ligands.

Conclusions: We solved the structure of the VPS39 C-terminal domain adopting a non-native fold. Our work highlights the risk of non-native folds when purifying small zinc-containing domains with hexahistidine tags. However, the non-native structure we observe may have implications for rational protein design.

Non-native fold of the putative VPS39 zinc finger domain

Background: The multi-subunit homotypic fusion and vacuole protein sorting (HOPS) membrane-tethering complex is involved in regulating the fusion of late endosomes and autophagosomes with lysosomes in eukaryotes. The C-terminal regions of several HOPS components have been shown to be required for correct complex assembly, including the C-terminal really interesting new gene (RING) zinc finger domains of HOPS components VPS18 and VPS41. We sought to structurally characterise the putative C-terminal zinc finger domain of VPS39, which we hypothesised may be important for binding of VPS39 to cellular partners or to other HOPS components. Methods: We recombinantly expressed, purified and solved the crystal structure of the proposed zinc-binding region of VPS39. Results: In the structure, this region forms an anti-parallel β-hairpin that is incorporated into a homotetrameric eight-stranded β-barrel. However, the fold is stabilised by coordination of zinc ions by residues from the purification tag and an intramolecular disulphide bond between two predicted zinc ligands. Conclusions: We solved the structure of the VPS39 C-terminal domain adopting a non-native fold. Our work highlights the risk of non-native folds when purifying small zinc-containing domains with hexahistidine tags. However, the non-native structure we observe may have implications for rational protein design.

A Novel Particulate Delivery System Based on Antigen-Zn2+ Coordination Interactions Enhances Stability and Cellular Immune Response of Inactivated Foot and Mouth Disease Virus

The interactions between antigen and adjuvant were among the most significant factors influencing the immunogenicity of vaccines, especially for unstable antigens like inactivated foot and mouth disease virus (iFMDV). Here we propose a novel antigen delivery pattern based on the coordination interaction between transition metal ions Zn²⁺ chelated to chitosan nanoparticles and iFMDV, which is known to be rich in histidine. The zinc chelated chitosan particles (CP-PEI-Zn) were prepared by cross-linking chitosan particles (CP) with sodium tripolyphosphate (TPP), modifying with metal chelator polyethylenimine (PEI), and subsequent chelating of Zn²⁺. The coordination interaction was confirmed by analyzing the adsorption and desorption behavior of iFMDV on CP-PEI-Zn by high-performance size exclusion chromatography (HPSEC), while the CP-PEI without chelating Zn²⁺ loads iFMDV mainly through electrostatic interactions. The iFMDV loaded on CP-PEI-Zn showed better thermal stability than that on CP-PEI, as revealed by a slightly higher transition temperature (T_m) related to iFMDV dissociation. After subcutaneous immunization in female Balb/C mice, antigens loaded on CP-PEI and CP-PEI-Zn all induced higher specific antibody titers, better activation of B lymphocytes, and more effector-memory T cells proliferation than the free antigen and iFMDV adjuvanted with ISA 206 emulsion did. Moreover, CP-PEI-Zn showed superior efficacy to CP-PEI in promoting the proliferation of effector-memory T cells and secretion of cytokines, indicating a more potent cellular immune response. In summary, the CP-PEI-Zn stabilized the iFMDV after loading and promoted both humoral and cellular immune responses, thus reflecting its potential to be a promising adjuvant for the iFMDV vaccine and other unstable viral antigens.

Deciphering metal ion preference and primary coordination sphere robustness of a designed zinc finger with high-resolution mass spectrometry

Small zinc finger (ZnF) motifs are promising molecular scaffolds for protein design owing to their structural robustness and versatility. Moreover, their characterization provides important insights into protein folding in general. ZnF motifs usually possess an exceptional specificity and high affinity towards Zn(II) ion to drive folding. While the Zn(II) ion is canonically coordinated by two cysteine and two histidine residues, many other coordination spheres also exist in small ZnFs, all having four amino acid ligands. Here we used high-resolution mass spectrometry to study metal ion binding specificity and primary coordination sphere robustness of a designed zinc finger, named MM1. Based on the results, MM1 possesses high specificity for zinc with sub-micromolar binding affinity. Surprisingly, MM1 retains metal ion binding affinity even in the presence of selective alanine mutations of the primary zinc coordinating amino acid residues.

Designing peptide sequences in flexible chain conformations to bind RNA: a search algorithm combining Monte Carlo, self-consistent mean field and concerted rotation techniques

A search algorithm combining Monte Carlo, self-consistent mean field, and concerted rotation techniques was developed to discover peptide sequences that are reasonable HIV drug candidates due to their exceptional binding to human tRNAUUU(Lys3), the primer of HIV replication. The search algorithm allows for iteration between sequence mutations and conformation changes during sequence evolution. Searches conducted for different classes of peptides identified several potential peptide candidates. Analysis of the energy revealed that the asparagine and cysteine at residues 11 and 12 play important roles in "recognizing" tRNA(Lys3) via van der Waals interactions, contributing to binding specificity. Arginines preferentially attract the phosphate linkage via charge-charge interaction, contributing to binding affinity. Evaluation of the RNA/peptide complex's structure revealed that adding conformation changes to the search algorithm yields peptides with better binding affinity and specificity to tRNA(Lys3) than a previous mutation-only algorithm.

Using the folding landscapes of proteins to understand protein function

Proteins fold on a biologically-relevant timescale because of a funnel-shaped energy landscape. This landscape is sculpted through evolution by selecting amino-acid sequences that stabilize native interactions while suppressing stable non-native interactions that occur during folding. However, there is strong evolutionary selection for functional residues and these cannot be chosen to optimize folding. Their presence impacts the folding energy landscape in a variety of ways. Here, we survey the effects of functional residues on folding by providing several examples. We then review how such effects can be detected computationally and be used as assays for protein function. Overall, an understanding of how functional residues modulate folding should provide insights into the design of natural proteins and their homeostasis.

Biochemical, Kinetic, and Spectroscopic Characterization of Ruegeria pomeroyi DddW--A Mononuclear Iron-Dependent DMSP Lyase

The osmolyte dimethylsulfoniopropionate (DMSP) is a key nutrient in marine environments and its catabolism by bacteria through enzymes known as DMSP lyases generates dimethylsulfide (DMS), a gas of importance in climate regulation, the sulfur cycle, and signaling to higher organisms. Despite the environmental significance of DMSP lyases, little is known about how they function at the mechanistic level. In this study we biochemically characterize DddW, a DMSP lyase from the model roseobacter Ruegeria pomeroyi DSS-3. DddW is a 16.9 kDa enzyme that contains a C-terminal cupin domain and liberates acrylate, a proton, and DMS from the DMSP substrate. Our studies show that as-purified DddW is a metalloenzyme, like the DddQ and DddP DMSP lyases, but contains an iron cofactor. The metal cofactor is essential for DddW DMSP lyase activity since addition of the metal chelator EDTA abolishes its enzymatic activity, as do substitution mutations of key metal-binding residues in the cupin motif (His81, His83, Glu87, and His121). Measurements of metal binding affinity and catalytic activity indicate that Fe(II) is most likely the preferred catalytic metal ion with a nanomolar binding affinity. Stoichiometry studies suggest DddW requires one Fe(II) per monomer. Electronic absorption and electron paramagnetic resonance (EPR) studies show an interaction between NO and Fe(II)-DddW, with NO binding to the EPR silent Fe(II) site giving rise to an EPR active species (g = 4.29, 3.95, 2.00). The change in the rhombicity of the EPR signal is observed in the presence of DMSP, indicating that substrate binds to the iron site without displacing bound NO. This work provides insight into the mechanism of DMSP cleavage catalyzed by DddW.

Understanding the folding-function tradeoff in proteins

When an amino-acid sequence cannot be optimized for both folding and function, folding can get compromised in favor of function. To understand this tradeoff better, we devise a novel method for extracting the "function-less" folding-motif of a protein fold from a set of structurally similar but functionally diverse proteins. We then obtain the β-trefoil folding-motif, and study its folding using structure-based models and molecular dynamics simulations. CompariA protein sequence serves two purpson with the folding of wild-type β-trefoil proteins shows that function affects folding in two ways: In the slower folding interleukin-1β, binding sites make the fold more complex, increase contact order and slow folding. In the faster folding hisactophilin, residues which could have been part of the folding-motif are used for function. This reduces the density of native contacts in functional regions and increases folding rate. The folding-motif helps identify subtle structural deviations which perturb folding. These may then be used for functional annotation. Further, the folding-motif could potentially be used as a first step in the sequence design of function-less scaffold proteins. Desired function can then be engineered into these scaffolds.

Connecting the kinetics and energy landscape of tRNA translocation on the ribosome

Functional rearrangements in biomolecular assemblies result from diffusion across an underlying energy landscape. While bulk kinetic measurements rely on discrete state-like approximations to the energy landscape, single-molecule methods can project the free energy onto specific coordinates. With measures of the diffusion, one may establish a quantitative bridge between state-like kinetic measurements and the continuous energy landscape. We used an all-atom molecular dynamics simulation of the 70S ribosome (2.1 million atoms; 1.3 microseconds) to provide this bridge for specific conformational events associated with the process of tRNA translocation. Starting from a pre-translocation configuration, we identified sets of residues that collectively undergo rotary rearrangements implicated in ribosome function. Estimates of the diffusion coefficients along these collective coordinates for translocation were then used to interconvert between experimental rates and measures of the energy landscape. This analysis, in conjunction with previously reported experimental rates of translocation, provides an upper-bound estimate of the free-energy barriers associated with translocation. While this analysis was performed for a particular kinetic scheme of translocation, the quantitative framework is general and may be applied to energetic and kinetic descriptions that include any number of intermediates and transition states.

A New Quantum Calibrated Force Field for Zinc-Protein Complex

A quantum calibrated polarizable-charge transfer force field (QPCT) has been proposed to accurately describe the interaction dynamics of zinc-protein complexes. The parameters of the QPCT force field were calibrated by quantum chemistry calculation and capture the polarization and charge transfer effect. QPCTs are validated by molecular dynamic simulation of the hydration shell of the zinc ion, five proteins containing the most common zinc-binding sites (ZnCys2His2, ZnCys3His1, ZnCys4, Zn2Cys6), as well as protein-ligand binding energy in zinc protein MMP3. The calculated results show excellent agreement with the experimental measurement and with results from QM/MM simulation, demonstrating that QPCT is accurate enough to maintain the correct structural integrity of the zinc binding pocket and provide accurate interaction dynamics of the zinc-residue complex. The current approach can also be extended to the study of interaction dynamics of other metal-containing proteins by recalibrating the corresponding parameters to the specific complexes.

Biomolecular dynamics: order-disorder transitions and energy landscapes

While the energy landscape theory of protein folding is now a widely accepted view for understanding how relatively weak molecular interactions lead to rapid and cooperative protein folding, such a framework must be extended to describe the large-scale functional motions observed in molecular machines. In this review, we discuss (1) the development of the energy landscape theory of biomolecular folding, (2) recent advances toward establishing a consistent understanding of folding and function and (3) emerging themes in the functional motions of enzymes, biomolecular motors and other biomolecular machines. Recent theoretical, computational and experimental lines of investigation have provided a very dynamic picture of biomolecular motion. In contrast to earlier ideas, where molecular machines were thought to function similarly to macroscopic machines, with rigid components that move along a few degrees of freedom in a deterministic fashion, biomolecular complexes are only marginally stable. Since the stabilizing contribution of each atomic interaction is on the order of the thermal fluctuations in solution, the rigid body description of molecular function must be revisited. An emerging theme is that functional motions encompass order-disorder transitions and structural flexibility provides significant contributions to the free energy. In this review, we describe the biological importance of order-disorder transitions and discuss the statistical-mechanical foundation of theoretical approaches that can characterize such transitions.

Connecting energy landscapes with experimental rates for aminoacyl-tRNA accommodation in the ribosome

Using explicit-solvent simulations of the 70S ribosome, the barrier-crossing attempt frequency was calculated for aminoacyl-tRNA elbow-accommodation. In seven individual trajectories (200-300 ns, each, for an aggregate time of 2.1 μs), the relaxation time of tRNA structural fluctuations was determined to be ∼10 ns, and the barrier-crossing attempt frequency of tRNA accommodation is ∼1-10 μs(-1). These calculations provide a quantitative relationship between the free-energy barrier and experimentally measured rates of accommodation, which demonstrate that the free-energy barrier of elbow-accommodation is less than 15 k(B)T, in vitro and in vivo.

Zinc-finger hydrolase: Computational selection of a linker and a sequence towards metal activation with a synthetic αββ protein

The zinc-finger protein is targeted for computational redesign as a hydrolase enzyme. Successful in having zinc activated for hydrolase function, the study validates the stepwise approach to having the protein tuned in main-chain structure stereochemically and over side chains chemically. A leucine homopolypeptide, harboring histidines to tri coordinate zinc and d-amino-acid-nucleated α-helix and β-hairpin building blocks of an αββ protein, is taken up for modeling, first with cyana, in a mixed-chirality linker between the building blocks, and then with IDeAS, in a sequence over side chains. The designed mixed-chirality polypeptide structure is proven to order as an intended αββ fold and capture zinc to activate its role as a hydrolase catalyst. The design approach to have protein folds defined stereochemically and receptor and catalysis functions defined chemically is presented, and illustrates L- and D-α-amino-acid structures as the alphabet integrating chemical- and stereochemical-structure variables as its letters.