Exploring how protamine folds DNA in sperm

DNA compaction is crucial for the transmission of genetic information. In sperm, the protein protamine binds to the major groove of DNA and compacts the DNA more tightly than histones. The more highly folded the DNA, the more hydrodynamic the sperm and the more likely it will reach the oocyte without UV damage. To understand the mechanism by which protamine packs DNA, we used an in vitro tethered particle motion assay and video microscopy to visualize the folding of 25 nm DNA. Because this length is shorter than the diameter of a DNA-protamine loop, we did not expect the DNA to fold. However, when we graphed the standard deviation of beads' motion over time, we found that the standard deviation decreases as protamine concentration increases. Our results suggest that protamine can fold short-length DNA molecules. Understanding the process by which this interaction occurs has applications in male infertility, epigenetics, and nanoengineering.