Substrate Identification Using a Chemical Genetics Approach Reveals a Role for PARP-7-Mediated MARylation in Controlling Microtubule Stability in Ovarian Cancer Cells
ADP-ribosylation (ADPRylation) is a regulatory posttranslational modification of proteins that results in the covalent attachment of ADP-ribose (ADPR) moieties on target proteins. PARP-7 (TiPARP) is a monoADPR transferase whose proteins substrates and biological activities are poorly understood. Here, we describe a chemical genetics approach that has allowed us to identify substrates of PARP-7 and explore their biological functions. We observed reduced levels of PARP7 mRNA in ovarian cancer patient samples versus normal tissue, but found nonetheless that PARP-7 is required for a number of cancer-related biological endpoints in ovarian cancer cells, including growth, migration, and invasion. Global gene expression and gene ontology analyses in ovarian cancer cells subjected to siRNA-mediated knockdown of PARP7 revealed an enrichment of genes encoding proteins with roles in cell-cell adhesion, cell cycle arrest, apoptosis, and gene regulation. To identify the ADPRylated substrates that underlie PARP-7-mediated ovarian cancer cell phenotypes, we developed an NAD+ analog-sensitive approach for PARP-7 comprising a PARP-7 NAD+ binding pocket mutant (S563G) paired with the NAD+ analog 8-Bu(3-yne)T-NAD+. When coupled with mass spectrometry, this approach allowed us to identify the PARP-7 ADP-ribosylated proteome in ovarian cancer cells, including components of the cell-cell adhesion and cytoskeleton organization machinery. Specifically, we found that PARP-7 monoADPRylates a-tubulin to promote microtubule instability, which may play a key role in the regulating ovarian cancer cell growth and motility. Collectively, our results point to an extensive PARP-7 ADP-ribosylated proteome with important roles in cancer-related cellular phenotypes. Overall design: Using RNA-seq we evaluated the expression changes of mRNAs in OVCAR4 cells after siRNA mediated PARP7 knockdown.