Researchers at the Institute of Structural Biology at Penn Medicine are using cryogenic electron microscopy (cryoEM) and cryo-electron tomography (cryoET) to advance understanding of biological processes. These technologies allow scientists to closely examine flash-frozen samples, providing new insights into how life functions at a molecular level.
Kathryn Kixmoeller, an MD/PhD student at the Perelman School of Medicine, has focused her research on mitosis, the process by which a cell divides. She and her colleagues used cryoEM to directly visualize the centromere region on intact chromosomes. This region joins each pair of duplicated chromosomes and plays a critical role in ensuring that genetic material is evenly distributed between two new cells during division. Kixmoeller’s work was co-advised by Yi-Wei Chang, assistant professor and associate director of the Institute of Structural Biology, and Ben Black, professor of Biochemistry and Biophysics.
Images produced by their team show that as chromosomes prepare to divide, chromosomal material forms a clearing around the centromere. This allows microtubule spindles access to kinetochore protein complexes so they can separate the X-shaped chromosome into two halves for equal distribution into new cells.
Research in Ben Black’s laboratory also explores epigenetics—the transmission of heritable information through gene or chromosome modifications rather than DNA sequence alone. Kenji Murakami, assistant professor of Biochemistry and Biophysics, collaborated with Marmorstein’s group to study histone proteins responsible for organizing DNA within chromosomes. Their teams used cryoEM single particle analysis to create high-resolution images showing how chaperone complexes help position histones in chromatin—a structure essential for DNA replication and repair.
Marmorstein explained: “This dedicated machine inserts specific histones, and the structure provided one snapshot of the initial stage of that insertion. What Kenji’s lab and our lab are now trying to do is capture later stages to get a more dynamic view of the entire insertion process.”
Roberto Dominguez, William Maul Measey Presidential Professor of Physiology II at Penn Medicine, emphasized that understanding protein structures is key to learning about their function: “Without knowing the structure of many of these proteins, it's very difficult to understand how they work, how they function, how they do what they do.”
Dominguez’s research focuses on actin proteins that form part of the cell's internal skeleton or cytoskeleton. His team used cryoEM in 2023 to visualize actin filament ends for the first time—an important step toward understanding muscle contraction, cell migration, cancer metastasis, and diseases caused by cytoskeletal mutations.
Graduate student Nicholas Palmer helped reveal how actin filaments grow and shorten by using artificially shortened filaments suitable for cryoEM imaging. The resulting images allowed researchers to create a movie depicting monomer addition—a process involving helper proteins such as formin and profilin. The findings were published in Nature in 2024; further studies aim to capture how actin strands shorten.
Kixmoeller summarized this approach: “Being able to visualize protein structures at such a stunning resolution, you can answer questions you don't even know how to ask before you start the process.”
Researchers believe visualizing these structures will continue driving progress in both fundamental biology and clinical science.
