IPiB Thesis Defense May 17, 2024: Sarah McMillan

Sarah McMillanSarah McMillan, an IPiB graduate student, will be defending her Ph.D. research on May 17, 2024. Her research in the Keck Lab investigated how the replicative helicase — a molecule that kicks off DNA replication in bacteria — is loaded onto DNA.

Replicative helicases, which are comprised of six subunits arranged in a ring formation, are responsible for unwinding DNA so that the DNA can be copied prior to cell division. Because of their ringed shape, replicative helicases are unable to load onto DNA on their own. They require “loader proteins,” which create a small opening, or crack, in the helicase ring, allowing the DNA to interact with the inner portion of the ring.

McMillan found that different structural variations in the loader protein cause the replicative helicase to load onto different DNA substrates . “By making small point mutations in the helicase loader,” explains McMillan, “we can alter what types of DNA substrates the helicase can be loaded on to.”

She also identified that the crack formed by loader proteins is used by replication restart proteins, which help to restore DNA replication if the process goes awry and is abandoned. Using cryo-electron microscopy (cryo-EM) in collaboration with the Grant Lab, McMillan and her colleagues visualized the structure of the E. coli replicative helicase-loader complex with a replication restart protein known as PriC. PriC binds at the opening in the replicative helicase/loader complex, helping to facilitate it loading onto DNA.

McMillan says that expanding our knowledge of the proteins responsible for loading helicases to DNA substrates can have therapeutic implications. “These replication restart proteins are essential in bacteria, but not in higher eukaryotes. So understanding how they facilitate loading replicative helicase onto DNA could give us information that would be beneficial in designing antibiotics.” Her research has been published in the Journal of Biological Chemistry.

When she joined the IPiB program, McMillan knew she was interested in studying structural biochemistry. She chose the Keck Lab because she was drawn to both the research and the supportive and collaborative environment. “Not all experiments will work,” reflects McMillan. “And that can be frustrating. So, it’s really important that you’re working with people who you enjoy being around, who will help you and support you and problem solve.”

McMillan has especially enjoyed opportunities to learn with and from her peers and undergraduate students. She has worked with the Research Experience for Undergraduates (REU) Program as a summer mentor and is currently a WISCIENCE Scientific Teaching Fellow. She will complete her teaching fellowship over the next year.

To learn more about McMillan’s research, attend her Ph.D. defense, “Structural and Biochemical Mechanisms of Replicative Helicase Loading in E. coli,” on Friday, May 17 at 10:00 a.m. CT in Room 1211 of Hector F. DeLuca Biochemical Sciences Building.