Deep in the crystalline chill of a cryo-electron microscopy lab at UT Southwestern Medical Center in Dallas, scientists have finally seen something no one has seen before: the exact shape of a protein that could unlock decades of failed vaccine efforts against chlamydia, the world's most common bacterial sexually transmitted infection.
The breakthrough matters because chlamydia is everywhere and invisible. An estimated 150 million people worldwide carried Chlamydia trachomatis infections in 2023, the latest year CDC data covers, yet many of them had no symptoms at all. They unknowingly passed the infection to partners and, in some cases, developed severe complications—infertility, chronic pain, blindness—years later. A vaccine could interrupt this silent spread, but researchers have been chasing one unsuccessfully for decades.
The key to that vaccine lay in a single protein called MOMP, the major outer membrane protein that sits on the surface of the bacterium's infectious stage. Scientists knew MOMP triggered immune responses, but when they tried building vaccines with denatured or recombinant versions of it—essentially broken versions lacking the protein's natural three-dimensional shape—the vaccines failed. This suggested that the protein's true structure was essential. The problem was no one had ever actually seen what that structure looked like.
Enter cryo-electron microscopy, or cryo-EM, a technique that freezes biomolecules at temperatures below -170°C and photographs them with an electron beam, revealing atomic-level detail. Dominika Borek, a professor of biophysics and biochemistry at UT Southwestern, and Luis M. de la Maza, a distinguished professor of pathology at the University of California, Irvine, led the team in imaging MOMP from Chlamydia muridarum, a species that infects mice. They captured the protein both alone and bound to an antibody fragment, building the first true picture of how this molecule actually appears in nature.
What emerged was unexpected. MOMP consists of three barrel-shaped units, each topped with a distinctive dome called an "antigenic cap." This cap is where the protein interacts with antibodies and host cells—the very place where immunity begins. The cap also contains variable domains, regions that shift between different chlamydia strains and determine which cells the bacteria can attach to, dictating what kind of infection develops. Understanding these variable regions is crucial because it means any vaccine will need to account for multiple strains.
The findings overturned a long-held assumption, too. Researchers had theorized that MOMP functioned as a molecular pore, allowing molecules to pass through the bacterial membrane. Instead, the structure revealed something different: the protein base acts as a stopper while the cap acts as a lid, completely sealing the membrane. The protein isn't a door; it's a guard.
Borek and her colleagues, including scientists from Lawrence Livermore National Laboratory, published their work in Nature Communications. "Many people are asymptomatic yet can still transmit the infection and later develop serious complications," Borek said. "This study provides a structural framework that could aid in the design of more effective vaccine antigens."
The implications extend beyond humans. Chlamydia pecorum infections are driving koalas toward extinction in Australia—another reminder that solving this puzzle has consequences across species. For the first time, researchers have a structural blueprint to work from.