Functional Annotation of Uncharacterized Enzymes in Yeast
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Functional Annotation of Uncharacterized Enzymes in Saccharomyces cerevisiae by Julia Ann Hanchard A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Department of Molecular Genetics University of Toronto © Copyright by Julia Hanchard 2019 Abstract Functional Annotation of Uncharacterized Enzymes in Saccharomyces cerevisiae Julia Hanchard Doctor of Philosophy, 2019 Department of Molecular Genetics University of Toronto In the post-genomic era, clinicians and scientists are increasingly reliant on interpretation of variants in metabolic genes for determining pathogenicity. These interpretations depend on functional annotation of the roles genes provide in metabolism, an annotation that is far from complete. I embarked on a journey of enzyme discovery to fill gaps in our knowledge of metabolism in the budding yeast, Saccharomyces cerevisiae. I carried out a genetic and metabolomic screen of 120 uncharacterized candidate enzyme encoding genes that comprised my master’s thesis. This dissertation describes my work in ascribing function to two distinct enzymes, Das2 and Tda5. Throughout my study I have found that Das2 is a novel uridine/cytidine kinase that functions in concert with a second minor uridine kinase, Urk1. These two enzymes are interdependent and in turn depend on a third enzyme, the major uracil phosphoribosyl transferase, Fur1 for stability. These three enzymes form a complex that is essential to wild-type pyrimidine salvage. As I aimed to elucidate the function of Tda5, I discovered that this uncharacterized enzyme is essential to growth. Loss of function mutations in TDA5 are alleviated by de-repression of its sporulation specific paralog, Ydl114w, and when YDL114W is deleted, tda5Δ is rescued by hypomorphic mutations in the ergosterol biosynthetic pathway. Analysis of metabolite levels in hand with nutritional sensitivities and genetic interaction data support the essential role of Tda5 in lipid homeostasis. Together, these studies assign new functions to previously uncharacterized genes and reveal functional interdependence of enzymes that sheds light on metabolic interpretations of genetic data. ii Acknowledgements I’m grateful to my supervisor, Dr. Amy Caudy, for her leadership at the helm of a multi- faceted and complex metabolomics lab. Her support and insight were crucial to the projects that comprise this dissertation. Thanks to my graduate advisory committee members Jim Dennis and Andy Bognar for their expert guidance through experiments and refining the trajectory of my thesis. I’d like to thank the Caudy lab tribe: Dr. Olga Zaslaver, Dr. Soumaya Zlitni, Yutong Ma and Yoomi Oh for their steady provision of motivation, support and friendship. I am especially grateful to my awesome team of undergraduate students who I’ve had the pleasure and privilege of working with: Muhammad Bin Munim, Connie Liu, Mashiat Khan, Sheena Faye Garcia, Lily Chen, Angela Dennisova, Sara Allarhakia and Jonathan Ward. Each of these students has provided technical work critical to this thesis, and it’s been a lot of fun in training them. I’ll always be indebted to my family for their unwavering love and support. Above all, I am thankful to Kasra Zokaei for his love, companionship and wise advice throughout my PhD. iii Table of Contents Acknowledgments.....................................................................................................................ii List of Tables............................................................................................................................ix List of Figures ..........................................................................................................................x List of Abbreviations...............................................................................................................xii 1 Introduction ...................................................................................................................... 1 1.1 Our knowledge of metabolism is incomplete .............................................................. 1 1.1.1 Early study of metabolism ................................................................................... 1 1.1.2 Clinical interpretation of genomes requires more study of metabolism .............. 1 1.1.3 Selective pressures lead to evolution of metabolic pathways and genome expansion ........................................................................................................................... 2 1.1.4 Prediction of enzyme function ............................................................................. 4 1.1.5 Prediction of enzyme substrates ........................................................................... 5 1.1.6 Domains of unknown function............................................................................. 6 1.1.7 Metabolic networks contain missing enzymes .................................................... 6 1.1.8 Metabolic networks are missing metabolites ....................................................... 7 1.1.9 New metabolic pathway information from fluxomics ......................................... 7 1.1.10 Enzyme discovery through metabolomic screening in yeast ............................... 8 1.2 Pyrimidine metabolism of Saccharomyces cerevisiae .............................................. 10 1.2.1 UMP is the central pyrimidine metabolite ......................................................... 10 1.2.2 de novo biosynthesis of UMP ............................................................................ 10 1.2.3 Regulation of UMP biosynthesis ....................................................................... 12 1.2.4 Multi-phosphorylated pyrimidine nucleotides ................................................... 13 1.2.5 Catabolism as a source of 5’ pyrimidine nucleotide monophosphates .............. 13 1.2.6 During autophagy 3’ pyrimidine nucleotides are hydrolyzed to nucleosides .... 14 1.2.7 Pyrimidine salvage from an uptake perspective ................................................ 15 1.2.8 Pyrimidine salvage enzymes- Early studies reveal pathway architecture and activities ........................................................................................................................... 15 1.2.9 Pyrimidine salvage - Purification of enzymes and cloning of genes ................. 16 1.2.10 Project rationale: Resolving pyrimidine salvage in yeast .................................. 17 1.3 Background for Tda5: a novel short chain dehydrogenase essential for respiration 17 1.3.1 Oxidoreductases and short chain dehydrogenases ............................................. 18 iv 1.3.2 SDR activity pervades lipid homeostasis ........................................................... 18 1.3.3 Membrane fluidity and structural integrity ........................................................ 23 1.3.4 Yeast growth is supported by fermentation and respiration .............................. 24 1.3.5 Compartmentalization of metabolism in yeast .................................................. 25 1.3.6 Compartmental proteomes are distinct .............................................................. 26 1.3.7 Project rationale: Elucidating the functions of Tda5 ......................................... 28 2 Methods ........................................................................................................................... 29 2.1 Methods relating to Chapter 3: Uncovering the function of Das2 ............................ 29 2.1.1 Growth and maintenance of yeast cultures ........................................................ 29 2.1.2 Construction of Mutants .................................................................................... 29 2.1.3 Preparation of point mutants by in vivo site directed mutagenesis .................... 29 2.1.4 HA fusion tagged alleles .................................................................................... 30 2.1.5 Heterologous expression of enzymes................................................................. 30 2.1.6 Preparation of Das2 G17E ................................................................................. 31 2.1.7 Radiometric enzyme assays ............................................................................... 31 2.1.8 Pyrimidine analogue resistance assays .............................................................. 32 2.1.9 Metabolite extracts ............................................................................................. 32 2.1.10 In vivo tracking of uridine kinase activity.......................................................... 32 2.1.11 Western Blot ...................................................................................................... 32 2.1.12 Preparation of Ribose-1-phosphate .................................................................... 33 2.2 Methods for Chapter 4: Phenotyping and mapping suppressors of Tda5 ................. 33 2.2.1 Growth and maintenance of yeast cultures ........................................................ 33 2.2.2 Construction of Mutants .................................................................................... 33 2.2.3 Spot assays ......................................................................................................... 35 2.2.4 Metabolite extracts ............................................................................................. 35 2.2.5 Purification of short and medium chain acyl-CoA metabolites