Trainee Spotlight: Sajitha Anthony, PhD
Sajitha Anthony, PhD
Former graduate student in
Dr. Jeffrey Peterson’s Lab
One might say that my educational journey beings in the North and ends in the South. I grew up in Toronto, the capitol of Ontario, Canada. Coming from a family of medical doctors, nurses, and scientists, my innate interests in medicine and biology were fostered at an early age. As a child, I was extremely interested in the medical field; I would often pore through my mother’s nursing and medical textbooks with fascination, looking at the descriptions of the human body and various medical aliments. After moving south to Pennsylvania for my mom’s new job, I explored my interests in medicine when I enrolled in Widener University to pursue a degree in psychology. In 2005, I graduated from Widener University with a bachelor’s degree in psychology, and in 2011 from Arcadia University with a bachelor’s degree in Biology. After graduation, I was at a crossroads—I could become a medical doctor, or I could go to graduate school and have a career focused on research and teaching. After much consideration, I chose to go to graduate school, starting at Drexel University College of Medicine in 2011 and working on my thesis in Dr. Jeffrey Peterson’s lab at Fox Chase Cancer Center. My thesis project focused on understanding filament formation of human Inosine Monophosphate Dehydrogenase (IMPDH). During my time in Jeff’s lab, I mentored students and even got an opportunity to teach a College level biology lab. This sparked my interest in teaching undergraduates. After defending my Ph.D. in May 2017, I headed further south, this time to Australia! My ultimate career goal is to become a professor and teach at the undergraduate level.
An increasing number of non-cytoskeletal metabolic enzymes undergo reversible polymerization in response to nutrient stress. Human Inosine Monophosphate Dehydrogenase 2 (hIMPDH2)—a rate-limiting metabolic enzyme involved in purine nucleotide biosynthesis—forms filaments under purine-depleted conditions; however, it is not known what role the IMPDH2 filaments play in cells, if hIMPDH2 is even functional in filaments, and what is the ultimate confirmation of hIMPDH2 in filaments. Furthermore, since hIMPDH2 is overexpressed in several cancers and implicated in viral responses, having a deeper understanding of how the confirmation and activity of hIMPDH2 are regulated (via these filaments) is of clinical relevance. Therefore, to study hIMPDH2, I created mutants, which were unable to form filaments, and then assessed the impact on catalytic activity. I discovered that filamentous and non-filamentous hIMPDH2 both had similar catalytic activities suggesting that the filamentous form of hIMPDH2 is not necessarily for function. In conjunction with Dr. Justin Kollman’s lab at the University of Washington, we used electron microscopy to determine the ultrastructure of hIMPDH2 following the addition of ligands ATP, GTP, NAD+ and IMP in various quantities and combinations. We discovered that not only was hIMPDH2 capable of assembling into an “extended” filament structure, it was also capable of assembling into a “compressed” octamer conformation, and a “bent” octamer conformation. It was surprising to us that the hIMPDH2 filaments were so flexible and accommodating to these different conformations, suggesting the possibility that each of these confirmations have some sort of physiological importance. Each octamer in the extended conformation had a length of ~110 Å, while the octamers in the compressed conformations had a length of ~95 Å, and each octamer in the bent conformation was half-compressed and half-extended in shape. The binding of the substrate ligands, IMP or NAD, increased the proportion of the extended octamer filaments, while GTP increased the proportion of filaments consisting of compressed octamers.
Whether or not hIMPDH2 filaments had catalytic activity of even increased catalytic activity has been much debated within this field and the results of my work finally helped put that debate to rest. The formation of open octamer filaments does not affect the activity of hIMPDH2. So why does hIMPDH2 assemble into these filaments? It is known that IMPDH has functions that are separate from its enzymatic activity. For example, it is known to act as a transcriptional repressor, regulating the expression of a number of different genes. IMPDH has also been found to associate with polyribosomes, implying a possible role in translation. Therefore, it is possible that filament assembly is a means by which cell controls IMPDH’s involvement in these other activities. This possibility definitely warrants further exploration.
Sajitha A. Anthony, Anika L. Burrell, Matthew C. Johnson, Krisna C. Duong-Ly, Yin-Ming Kuo, Jacqueline C. Simonet, Peter Michener, Andrew Andrews, Justin M. Kollman, Jeffrey R. Peterson. Reconstituted IMPDH polymers accommodate both catalytically active and inactive conformations. Mol Biol Cell. 2017 Aug 9; E17-04-0263.