Structural And Functional Analysis Of Proteins With The Double Stranded β-helix (Cupin) Domains
Proteins performing catalytic roles predominantly occur in a few protein folds. Functional diversity within a common structural scaffold has been attributed to conformational features that enable exploration of reaction space. In this study, we examined specific aspects of functional diversity in the Double Stranded β-helix(cupin) fold. The cupin domain is a hyper-stable protein fold that can support a variety of functions. Variation in function using a conserved active site in the cupin fold is achieved by changes in the residues that line the active site cavity as well as by the choice of a metal cofactor. Although this appears to be a likely basis for functional diversification, a few exceptions exist. It is thus interesting to examine how enzymes with the same structure, metal cofactor and ligand coordination catalyze a diverse range of reactions. This thesis describes two bi-cupins, BacB (also known as bacilysin synthase, YwfC) and Quercetinase (YxaG). BacB is a part of the protein machinery involved in the synthesis of a di-peptide antibiotic bacilysin. The case of the bicupin protein BacB illustrates the problem of functional annotation of proteins with the cupin fold. None of the predicted functions for this enzyme could be experimentally validated in vitro. The crystal structure, determined by Single-wavelength Anomalous Dispersion (SAD) based on the bound metal-ion at the active site provided a basis to evaluate the catalytic role of this protein. Eventually, the function of this protein could be determined based on characterizing the gene product of bacA, the gene preceding bacB in the B. subtilis bac operon. The crystal structure determination of BacB also led to an analysis of multiple crystal forms, with implications for the role of molecular symmetry in forming protein crystals. The stability of the cupin domain was examined using B. subtilis quercetinase as a model system. The availability of the crystal structure and a robust activity assay enabled us to examine the role of fragment complementation in the stability of the cupin scaffold and its implications for the function of this enzyme. This thesis also has a section on the use of structural homology for function annotation for cupin proteins. The results presented here thus provide a frame-work to understand the structural basis for functional diversity in the cupin family. This thesis is organized as follows: Chapter 1: This chapter provides an introduction to the Double Stranded β-Helix-Helix (DSBH or cupin) fold. Proteins with a cupin scaffold are remarkably diverse - spanning both enzymatic and non-enzymatic functions. This chapter presents a compilation of previous reports encompassing eighteen different functional classes. These functions include seed storage, transcription factors and a host of various enzymatic activities. Cupin proteins can be monocupins, bicupins or multi-domain cupins based on the number of DSBH domains in a single polypeptide chain. Very few multi-domain cupin proteins have been identified and this is generally not considered to be a significant sub-group. The inference that cupin proteins with more than one domain are products of gene duplication events is also examined in detail. The latter part of this chapter aims to provide an introduction to the two model proteins B. subtilis BacB and Quercetinase. Chapter 2: This chapter describes studies on a bi-cupin protein BacB involved in bacilysin synthesis. Bacilysin is a non-ribosomally synthesized dipeptide antibiotic that is active against a wide range of bacteria and some fungi. Synthesis of bacilysin (L-alanine-[2,3-epoxycyclohexano-4]-L-alanine) is achieved by proteins in the bac operon, also referred to as the bacABCDE (ywfBCDEF) gene cluster in B. subtilis. The production of this antibiotic is regulated via a stringent response and branches off the pathway for aromatic amino-acid biosynthesis at prephenate. Extensive genetic analysis from several strains of B. subtilis suggests that the bacABC gene cluster encodes all the proteins that synthesize the epoxyhexanone ring of L-anticapsin. This data, however, could not be reconciled with the putative functional assignments for these proteins whereby BacA, a prephenate hydratase along with a potential isomerase/guanylyl transferase, BacB and an oxidoreductase, BacC, could synthesize L-anticapsin. Here, based on the characterization of the reaction products of BacA and BacB as well as the crystal structure of BacB, we demonstrate that B. subtilis BacB catalyzes the synthesis of 2-oxo-3-(4-oxocyclohexa-2,5-dienyl)propanoic acid, a precursor to L-anticapsin. The mass and NMR spectra of the reaction product of BacA suggest that BacA is a decarboxylase that acts on prephenate. BacB is an oxidase. This protein is a bi-cupin, with two putative active sites each containing a bound metal ion. Additional electron density at the active site of the C-terminal domain of BacB could be interpreted as a bound phenylpyruvicacid (PPY). A significant decrease in the catalytic activity of a point variant of BacB with a mutation at the N-terminal domain suggests that the N-terminal cupin domain is involved in catalysis. Chapter 3 is based on the crystal packing analysis of three different crystal forms of B. subtilis BacB. BacB is an oxidase that catalyzes the production of the di-peptide antibiotic bacilysin. This protein is a bi-cupin with two double stranded β-helix domains fused in a compact arrangement. BacB crystallizes in three crystal forms, belonging to the triclinic, monoclinic and tetragonal space groups. These different crystal forms could be obtained in similar crystallization conditions. We also note that a slight disturbance to the crystallization droplet results in nucleation events, eventually resulting in a different crystal form. All three crystal forms of BacB diffract to high resolution, thus enabling the structure determination and analysis of the packing arrangements of BacB in different space groups. Metal ions at the lattice interface dominate the different packing arrangements. The crystal packing reveals that a dimer of BacB serves as the template on which higher order symmetrical arrangements are formed. BacB, however, is a monomer in solution. The different crystal forms of BacB thus provide experimental evidence to the hypothesis that molecular symmetry could aid crystallization. Chapter 4 provides a conformational analysis of the cupin fold using B. subtilis quercetinase as a model system to understand the conformational determinants of functional diversity. Controlled proteolysis experiments revealed that this enzyme is active, thermo-stable and maintains its quaternary arrangement even after substantial (ca 33 %) cleavage of the protein. The results presented in this chapter thus show that the cupin scaffold offers a balance between protein stability and function by locating the active site and substrate recognition features in the most stable region of the protein. Chapter 5 is based on the phylogenetic analysis of cupin domains. The members of cupin superfamily exhibit large variations in their sequences, functions, organization of domains, quaternary association and the nature of bound metal ion despite having a conserved β-barrel structural scaffold. Here, an attempt was made to understand structure-function relationships among the members of this diverse superfamily and identify the principles governing functional diversity. The cupin superfamily also contains proteins for which structures are available through world-wide structural genomics initiatives but characterized as “hypothetical”. We have explored the feasibility of obtaining clues to functions of such proteins by means of comparative analysis with cupins of known structure and function. This phylogenetic strategy was applied to BacB leading to clustering with oxidoreductases. BacB was experimentally demonstrated to be an oxidase. Chapter 6 is a summary of the work reported in this thesis and the conclusions that can be drawn based on these studies. The appendix section of this thesis comprises additional experimental details, methodology and aspects of the techniques used in this study. Appendix I contains a description of a methodology for Molecular Replacement (MR) calculations in obtaining phase information for protein crystallography. Appendix II provides additional details of experimental protocols.
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