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Chlamydia protein has an odd structure, scientists find

Research could lead to new ways to combat this sexually transmitted disease

A protein secreted by the chlamydia bug has a very unusual structure, according to scientists in the School of Medicine at The University of Texas Health Science Center San Antonio. The discovery of the protein's shape could lead to novel strategies for diagnosing and treating chlamydia, a sexually transmitted disease that infects an estimated 2.8 million people in the U.S. each year.

The protein, Pgp3, is secreted by Chlamydia trachomatis, the bacterium that causes chlamydia. Pgp3's shape is very distinguishable — sort of like an Eiffel Tower of proteins. "From a structural standpoint, the protein is very odd indeed," said X-ray crystallographer P. John Hart, Ph.D., the Ewing Halsell President's Council Distinguished Chair in the Department of Biochemistry at the San Antonio medical school. "This long and slender molecule contains a fusion of structural motifs that resemble those typically found in viral and not bacterial proteins." Dr. Hart is co-lead author of the research, which is described in the Journal of Biological Chemistry (JBC).

The Pgp3 protein is a chlamydial virulence factor that is hypothesized to enhance the bug's ability to initially infect its human host and then evade host defenses. "Although my lab has worked on this protein for many years and gained a great deal of knowledge on it, we still don't know what roles it may play in chlamydial pathogenesis (disease development)," said co-lead author Guangming Zhong, M.D., Ph.D., professor of microbiology at the Health Science Center. "With the structural information uncovered in this paper, we can now test many hypotheses."

This is the second chlamydial virulence factor that Dr. Zhong's laboratory has identified; the first was a protein called CPAF. Structural studies have played an important role in understanding CPAF's functions in chlamydial infections, Dr. Zhong said.

Chlamydia's toll

According to the U.S. Centers for Disease Control and Prevention (CDC), more than 1.4 million new cases of chlamydia were reported in 2011 across the 50 states and the District of Columbia. But the CDC says as many cases go unreported because most people with chlamydia have no symptoms and do not seek testing. If left untreated, chlamydia can permanently damage a woman's reproductive system. This can lead to ectopic pregnancy, pelvic inflammatory disease and infertility.

The disease burden of chlamydia worldwide is magnitudes greater, with new cases numbering in the dozens of millions per year. The World Health Organization estimates that 499 million new cases occur annually of four curable sexually transmitted diseases — chlamydia, syphilis, gonorrhea and trichomoniasis. This estimate is for cases in adults aged 15-49.

Chlamydia infection induces inflammatory pathology in humans, and Pgp3 may contribute to the pathology by activating inflammation via one of its structural features uncovered in the crystal structure, said Dr. Zhong, who has worked with Dr. Hart on the Pgp3 project for nearly four years.

ACKNOWLEDGMENTS: This work was supported by Robert A. Welch Foundation grant AQ-1399 (to PJH), and National Institutes of Health (NIH)/National Institute of Allergy and Infectious Diseases grants AI47997 & AI64537 (to GZ). Portions of this work were supported by the Army Research Office of the U.S. Department of Defense under contract W911NF-11-1-0136 to The University of Texas at San Antonio/The University of Texas Health Science Center at San Antonio Center for Excellence in Genomics Research. A portion of the work took place at the Northeastern Collaborative Access Team beam lines of the Advanced Photon Source at Argonne National Laboratory, supported by award RR-15301 from the National Center for Research Resources at the NIH. Use of the Advanced Photon Source is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under contract W-31-109-ENG-38. Merck supported studies on the immunogenicity of Pgp3 in humans (to GZ). This work was also supported in part by the Cancer Center Support Grant of the Cancer Therapy & Research Center (CTRC) at the UT Health Science Center San Antonio (National Cancer Institute grant P30CA054174). Support for the X-ray Crystallography Core Laboratory by the UT Health Science Center San Antonio Office of the Vice President for Research is gratefully acknowledged.

Structure of the Chlamydia trachomatis Immunodominant Antigen Pgp3

Ahmad Galaleldeen 1,4,#, Alexander B. Taylor 1,3,#, Ding Chen 2, Jonathan P. Schuermann 5, Stephen P. Holloway 1, Shuping Hou 2, Siqi Gong 2, Guangming Zhong 2 and P. John Hart 1,3,6

1 Department of Biochemistry, 2 Department of Microbiology & Immunology, and 3 X-ray Crystallography Core Laboratory, University of Texas Health Science Center, San Antonio, TX 78229; 4 St. Mary's University, Department of Biological Sciences, One Camino Santa Maria, San Antonio, TX 78228; 5 Northeastern Collaborative Access Team, Department of Chemistry and Chemical Biology, Cornell University, Ithaca, NY 14853; 6 Geriatric Research, Education, and Clinical Center, Department of Veterans Affairs, South Texas Veterans Health Care System, San Antonio TX 78229

#These authors contributed equally to this work.

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The University of Texas Health Science Center at San Antonio, one of the country's leading health sciences universities, ranks in the top 3 percent of all institutions worldwide receiving National Institutes of Health funding. The university's schools of medicine, nursing, dentistry, health professions and graduate biomedical sciences have produced approximately 28,000 graduates. The $736 million operating budget supports eight campuses in San Antonio, Laredo, Harlingen and Edinburg. For more information on the many ways "We make lives better®," visit

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