Uniformed Services University of the Health Sciences
Department of Biochemistry and Molecular Biology
4301 Jones Bridge Road, C1094
Bethesda, Maryland 20814-4799
Fax: (301) 295-3512
Lab: (301) 295-3111
Research in my lab is to study potential drug target proteins encoded in the genome of Mycobacterium tuberculosis (MTB), the causative agent of the disease tuberculosis. We use X-ray crystallographic techniques and in vitro biochemical assays to analyze the functions, reaction mechanism, and protein-protein interactions of these proteins. Current projects include studying the PhoP-PhoR two-component signaling system, and an iron-regulated ABC-type transporter IrtAB.
The PhoP-PhoR two-component system is essential for virulence and intracellular growth of MTB. Global profiling of gene expression indicates that at least 44 genes are up-regulated and 70 genes are down-regulated by PhoP-PhoR. A mutant MTB lacking this two-component system has defects in cell envelope, and it cannot grow in human and mouse macrophages or in mice. PhoR is a sensor histidine kinase that transmits environmental signals through cell membrane by autophosphorylation on its cytosolic domain. The phosphate group is then transferred to PhoP, a response regulator, to activate its regulation of gene transcription. We are studying the structures of the PhoP and PhoR proteins, the interactions between them, the mechanism of PhoR autophosphorylation and phosphorylation of PhoP, and the DNA recognition mechanism of PhoP. This project is in close collaboration with Dr. Smith?s lab at the PHRI center, New Jersey Medical School, Newark, New Jersey.
PhoP has two distinct domains, an N-terminal regulatory domain (also called receiver domain) that contains the phosphorylation site aspartate and a C-terminal DNA-binding domain (also called effector domain). The crystal structure of PhoP indicates that it can dimerize through the receiver domain to form a symmetric dimer. The effector domains of the dimer are tethered to the N-terminal domains through a flexible linker, but do not have any interactions between themselves or with the receiver domains in the dimer. While the sequence recognition helix is exposed without phosphorylation activation and is able to bind DNA, phosphorylation is likely to stabilize the receiver domain dimer and thus increase binding affinity to tandem repeat DNA sequences by bringing two effector domains in close proximity.
PhoR has a modular domain structure: an extracytosolic sensor domain (PhoRE), a transmembrane domain (PhoRTM), and a cytosolic domain (PhoRC). The cytosolic domain can be subdivided into a HAMP domain (PhoRH), a dimerization domain (PhoRDD), and an ATPase domain (PhoRA). A truncated domain (PhoRK) containing both PhoRA and PhoRDD is expected to have the kinase activity. We are working on these truncated domains, as well as the full-length PhoR protein.
The genes irtA and irtB are regulated by iron and are in a single operon. IrtA and IrtB are predicted to form a heterodimeric ABC-type transporter essential for import of iron-siderophore complex under low iron conditions. There is an atypical extension of ~290 residues at the N-terminus of IrtA, which is predicted to bind siderophore based on sequence analysis. This extension is termed SID for siderophore interacting domain. SID expressed and purifed from E. coli has a bright yellow color from its bound cofactor FAD. Because SID is located in the cytosol, it is likely to function as a ferrireductase to release iron from its siderophore complex.
S. Menon and S. Wang. Structure of the response regulator PhoP from Mycobacterium tuberculosis reveals a dimer through the receiver domain. Biochemistry (2011), 50, 5948-5957.
M. B. Ryndak, S. Wang, I. Smith, and G. M. Rodriguez. The Mycobacterium tuberculosis high-affinity iron importer, IrtA, contains an FAD-binding Domain. J. Bacteriol. (2010) 192, 861-869.
M. Ryndak, S. Wang, and I. Smith. PhoP, a key player in Mycobacterium tuberculosis virulence. Trends in Microbiology (2008) 16, 861-869.
S. Wang, J. Engohang-Ndong, and I. Smith. Structure of the DNA-binding domain of the response regulator PhoP from Mycobacterium tuberculosis. Biochemistry (2007) 46, 14751-14761.
S. Wang and D. Eisenberg. Crystal Structure of the Pantothenate Synthetase from Mycobacterium tuberculosis, Snapshots of the Enzyme in Action. Biochemistry (2006) 45, 1554-1561.
M. Strong, M. Sawaya, S. Wang, M. Phillips, D. Cascio and D. Eisenberg. Toward the structural genomics of complexes: crystal structure of a PE/PPE protein complex from Mycobacterium tuberculosis. Proc Natl Acad Sci U S A. (2006) 103, 8060-8065.
S. Wang and D. Eisenberg. Crystal Structures of a Pantothenate Synthetase from M. tuberculosis and its complexes with substrates and a reaction intermediate. Protein Science (2003) 12, 1097-1108.
S. Wang, C. Mura, M. Sawaya, D. Cascio and D. Eisenberg. Crystal Structure of a Nudix Protein from Pyrobaculum aerophilum reveals a dimer with two intersubunit ? sheets. Acta Cryst. (2002) D58, 571-578.
S. Wang, L. Tabernero, M. Zhang, E. Harms, R. L. Van Etten and C. V. Stauffacher. Crystal Structures of a Low Molecular Weight Protein Tyrosine Phosphatase from Saccharomyces cerevisiae and its Complex with the Substrate p-Nitrophenyl Phosphate. Biochemistry (2000) 39, 1903-1914.
S. Wang, C. V. Stauffacher and R. L. Van Etten. Structural and Mechanistic Basis for the Activation of a Low Molecular Weight Protein Tyrosine Phosphatase by Adenine. Biochemistry (2000) 39, 1234-1242.