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Infection and Immunity, August 2008, p. 3415-3428, Vol. 76, No. 8
0019-9567/08/$08.00+0     doi:10.1128/IAI.01377-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.

The Chlamydial Plasmid-Encoded Protein pgp3 Is Secreted into the Cytosol of Chlamydia-Infected Cells{triangledown}

Zhongyu Li,1,2 Ding Chen,1 Youmin Zhong,1 Shiping Wang,2 and Guangming Zhong1*

Department of Microbiology and Immunology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, Texas 78229,1 Department of Parasitology, Xiangya Medical School, The Central South University, 168 Tongzipo Rd., Changsha, Hunan 410078, The People's Republic of China2

Received 12 October 2007/ Returned for modification 1 February 2008/ Accepted 7 May 2008

The chlamydial cryptic plasmid encodes eight putative open reading frames (ORFs), designated pORF1 to -8. Antibodies raised against these ORF proteins were used to localize the endogenous proteins during chlamydial infection. We found that the pORF5 protein (also known as pgp3) was detected mainly in the cytosol of Chlamydia-infected cells, while the remaining seven proteins were found inside the chlamydial inclusions only. The pgp3 distribution pattern in the host cell cytosol is similar to but not overlapping with that of chlamydial protease/proteasome-like activity factor (CPAF), a chlamydial genome-encoded protein known to be secreted from chlamydial inclusions into the host cell cytosol. The anti-pgp3 labeling was removed by preabsorption with pgp3 but not CPAF fusion proteins and vice versa, demonstrating that pgp3 is a unique secretion protein. This conclusion is further supported by the observation that pgp3 was highly enriched in cytosolic fractions and had a minimal presence in the inclusion-containing nuclear fractions prepared from Chlamydia-infected cells. The pgp3 protein was detected as early as 12 h after infection and was secreted by all chlamydial species that carry the cryptic plasmid, suggesting that there is a selection pressure for maintaining pgp3 secretion during chlamydial infection. Although expression of pgp3 in the host cell cytosol via a transgene did not alter the susceptibility of the transfected cells to the subsequent chlamydial infection, purified pgp3 protein stimulated macrophages to release inflammatory cytokines, suggesting that pgp3 may contribute to chlamydial pathogenesis.

* Corresponding author. Mailing address: Department of Microbiology and Immunology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, TX 78229. Phone: (210) 567-1169. Fax: (210) 567-0293. E-mail: Zhongg{at}UTHSCSA.EDU

{triangledown} Published ahead of print on 12 May 2008.

Editor: J. F. Urban, Jr.

Infection and Immunity, August 2008, p. 3415-3428, Vol. 76, No. 8
0019-9567/08/$08.00+0     doi:10.1128/IAI.01377-07
Copyright © 2008, American Society for Microbiology. All Rights Reserved.



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