Determination of Fatty Acid Synthesis Intermediates in Escherichia Coli and Bacillus Subtilis
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DETERMINATION OF FATTY ACID SYNTHESIS INTERMEDIATES IN ESCHERICHIA COLI AND BACILLUS SUBTILIS BY SWAMINATH SRINIVAS DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Biochemistry in the Graduate College of the University of Illinois at Urbana-Champaign, 2018 Urbana, Illinois Doctoral Committee: Professor John E. Cronan Jr., Chair Professor James A. Imlay Professor Satish K. Nair Professor Wilfred A. van der Donk ABSTRACT Lipids play crucial roles in maintaining cellular structure and energy storage. Structural lipids in the form of phospholipids constitute almost 10% of the dry cell weight in bacteria with their synthesis requiring 32 moles of ATP per mole of lipid. This significant investment ensures that the flux through the fatty acid biosynthesis (FAS) and related metabolic pathways is very precisely coordinated. A key feature of the FAS pathway is the acyl carrier protein (ACP), which is a small acidic protein that tethers acyl intermediates via a high-energy thioester bond and shuttles them between enzymes. In Escherichia coli, ACP is one of the most abundant soluble proteins with about 60000 copies per cell. Despite being the subject of extensive biochemical and structural studies for several decades, a reliable snapshot of ACP-bound species in any organism under different conditions is unavailable. Previously used methods are severely limited in their capacity to differentiate fatty acid intermediates, suffer from poor reproducibility, require elaborate instrumentation and cannot be used in an ideal setting for determining intracellular fluxes. This dissertation describes a sensitive and facile method to identify and quantify the physiological level of acyl-ACP species in E. coli and Bacillus subtilis under varying conditions of growth. The method primarily relies on an inert strep-tagged ACP which is purified using a two-step purification strategy utilizing volatile buffers of low pH in order to preserve the thioester bond between the acyl group and ACP. Samples obtained via this method were extremely pure and free of unwanted salts. The full length purified strep-tagged ACP were subjected to ESI mass spectrometry. ii Short and medium chain saturated fatty acyl-ACPs as well as long chain saturated and unsaturated fatty acyl-ACP intermediates were observed. Apart from validating widely held ideas about acyl- ACP flux in basic metabolism, acetyl-ACP, a novel intermediate, was also observed in abundance along with much lower levels of holo-ACP than previously thought. Current efforts are focusing on developing and validating a new model for the initiation of fatty acid biosynthesis using a combination of FAS mutants and stable-isotope labelling studies. During our efforts to construct one such mutant, we identified and characterized the function of a FabG temperature-sensitive mutant missing eight residues at its dimerization interface. Surprisingly, this mutant behaved like a point mutant. We identified interactions within the dimerization/tetramerization interface that compensate for each other and showed that the wildtype FabG predominantly exists as a dimer but functions as a tetramer. The mutation also rendered the enzyme extremely sensitive to calcium in vitro and conferred resistance to a calmodulin inhibitor in vivo. In B. subtilis, we have determined that the cellular levels of ACP are ten times lower than those in E. coli. when normalized to the total protein content per cell. Even under our stringent purification conditions, ACP co-purified with several other proteins, most noticeably FabF. We are currently trying to identify if Bacillus ACP is part of a loose complex. Nevertheless, this method has also successfully resolved all expected fatty acid intermediates associated with ACP including the presence of acetyl-ACP as observed in E. coli. iii In addition, I also discuss the development of a new set of vectors with IPTG-dependent origins of replication and a robust CRISPR-Cas9 toolkit for E. coli based on this vector set. iv ACKNOWLEDGEMENTS असतो मा सद्गमय । तमसो मा ज्योतत셍गमय । From ignorance, lead me to the truth. From darkness, lead me to the light I would like to start by thanking John Cronan, my advisor and my role model for his guidance and immense help over these many years. He has been patient and supportive throughout the course of my research and has given me the complete freedom to pursue my own scientific curiosities. His infectious passion and dedication to science has helped me define myself as a scientist. I would like to thank all the members of the Cronan lab, both past and present, for being wonderful colleagues, collaborators and friends. I would especially like to thank Quin Christensen, Alexander Smith, Youjun Feng and Steven Lin for getting me started in lab and for answering innumerable questions patiently. I thank my committee members, Dr. James Imlay, Dr. Satish Nair and Dr. Wilfred van der Donk for their helpful discussion of my work. I also thank Dr. Peter Yau and Dr. Brain Imai from the proteomics facility for helping me understand mass spectrometry and for making this project viable. Finally, I would like to extend a special thanks to my family, especially my parents and my wife for their constant encouragement, understanding and unwavering support from beginning to end. Without you, none of this would have been possible. v TABLE OF CONTENTS CHAPTER 1: INTRODUCTION .....................................................................................1 1.1 TYPE II FATTY ACID BIOSYNTHETIC REACTIONS ................................2 1.2 THE ACYL CARRIER PROTEIN ....................................................................5 1.3 INTRACELLULAR TURNOVER OF ACP PROSTHETIC GROUP .............7 1.4 COMPOSITION AND REGULATION OF THE INTRACELLULAR ACP POOL ..............................................................................................................8 1.5 THE INTERACTIVE NATURE OF ACP ......................................................10 1.6 FATTY ACID SYNTHESIS INITIATION AND ITS INHIBITORS ............11 1.7 BETA-KETOACYL-ACP REDUCTASE: FABG ..........................................12 1.8 AIMS AND SCOPE OF THIS THESIS ..........................................................14 1.9 FIGURES .........................................................................................................17 CHAPTER 2: IDENTIFICATION AND CHARACTERIZATION OF FATTY ACID SYNTHESIS INTERMEDIATES IN ESCHERICHIA COLI AND BACILLUS SUBTILIS ...........................................................................................22 2.1 INTRODUCTION ...........................................................................................22 2.2 MATERIALS AND METHODS .....................................................................25 2.2.1 Bacterial Strains, Plasmids and Materials .........................................25 2.2.2 Construction of Strep-tagged ACP ...................................................26 2.2.3 Growth Measurements ......................................................................27 2.2.4 In vitro Fatty Acid Synthesis Assay..................................................27 vi 2.2.5 Radioactive Labeling of Phospholipids and ACP .............................28 2.2.6 Purification of Strep-tagged ACP .....................................................29 2.2.7 Mass Spectral Analysis .....................................................................30 2.3 EXPERIMENTAL RESULTS.........................................................................31 2.3.1 Carboxy-terminal Strep-tagged ACP is Functional in vivo ..............31 2.3.2 Strep-tagged ACP is Easier to Measure and Visualize than its Native Form ...............................................................................................31 2.3.3 Strep-tagged ACPs are Functional in vitro .......................................32 2.3.4 E. coli has Five to Ten-fold more ACP than B. subtilis ....................32 2.3.5 The ACP Interactome of B. subtilis ..................................................33 2.3.6 FabF Tightly Associates with B. subtilis ACP in a Stoichiometric Fashion ..............................................................................33 2.3.7 Purification of Acyl-ACP species from E. coli and B. subtilis .........34 2.3.8 Mass Spectrometric Analysis and Validation ...................................35 2.3.9 The Metabolic Role of Acetyl-ACP .................................................36 2.4 DISCUSSION ..................................................................................................36 2.5 TABLES AND FIGURES ...............................................................................40 CHAPTER 3: AN EIGHT RESIDUE DELETION IN ESCHERICHIA COLI FABG CAUSES TEMPERATURE-SENSITIVE GROWTH AND LIPID SYNTHESIS PLUS RESISTANCE TO THE CALMODULIN INHIBITOR, TRIFLUOPERAZINE .....................................................................................................53 3.1 INTRODUCTION ...........................................................................................53 vii 3.2 MATERIALS AND METHODS .....................................................................55 3.2.1 Bacterial Strains, Plasmids and Materials .........................................55 3.2.2 Construction of E. coli MC1061 fabG∆8 using CRISPR/Cas9 ........56 3.2.3 Structural Modeling and Sequence Alignment .................................57 3.2.4 Expression and Purification of a His-tagged FabGΔ8 Protein