Human GCIP interacts with CT847, a novel Chlamydia trachomatis type III secretion substrate, and is degraded in a tissue-culture infection model

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Abstract
The obligate intracellular bacterium Chlamydia trachomatis occupies a parasitophorous vacuole and employs a type III secretion mechanism to translocate host-interactive proteins. These proteins most likely contribute to pathogenesis through modulation of host cell mechanisms crucial for the establishment and maintenance of a permissive intracellular environment. Using a surrogate Yersinia type III secretion system (T3SS), we have identified the conserved gene product CT847 as a chlamydial T3SS substrate. Yeast two-hybrid studies using CT847 as bait to screen a HeLa cell cDNA library identified an interaction with mammalian Grap2 cyclin D-interacting protein (GCIP). Immunoblot analyses of C. trachomatis-infected HeLa cells showed that GCIP levels begin to decrease (as compared with mock-infected HeLa cells) between 8 h and 12 h post infection. GCIP was virtually undetectable in 24 h time point material. This decrease was inhibited by proteasome inhibitors lactacystin and MG-132, and the T3SS inhibitor Compound 1. CT847 was detectible in purified reticulate body but not elementary body lysates, and reverse transcription polymerase chain reaction (RT-PCR) expression analyses indicate a mid-cycle expression pattern. Both of these findings are consistent with CT847 contributing to the observed effect on GCIP. Given the established roles of GCIP, we believe that we have discovered a novel C. trachomatis antihost protein whose activity is relevant to chlamydial pathogenesis.