Structural and Functional Insights into the Stealth Protein CpsY of Mycobacterium tuberculosis .

Mycobacterium tuberculosis ( Mtb ) is an important and harmful intracellular pathogen that is responsible for the cause of tuberculosis (TB). Mtb capsular polysaccharides can misdirect the host's immune response pathways, resulting in additional challenges in TB treatment. These capsule polysaccharides are biosynthesized by stealth proteins, including CpsY. The structure and functional mechanism of Mtb CpsY are not completely delineated. Here, we reported the crystal structure of CpsY 201-520 at 1.64 Å. CpsY 201-520 comprises three β-sheets with five α-helices on one side and three on the other. Four conserved regions (CR1-CR4) are located near and at the base of its catalytic cavity, and three spacer segments (S1-S3) surround the catalytic cavity. Site-directed mutagenesis demonstrated the strict conservation of R419 at CR3 and S1-S3 in regulating the phosphotransferase activity of CpsY 201-520 . In addition, deletion of S2 or S3 (∆S2 or ∆S3) dramatically increased the activity compared to the wild-type (WT) CpsY 201-520 . Results from molecular dynamics (MD) simulations showed that S2 and S3 are highly flexible. Our study provides new insights for the development of new vaccines and targeted immunotherapy against Mtb.