NGUYEN VAN SANG
Email Address
dbsnvs@nus.edu.sg
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Publication Structure of AcrH-AopB Chaperone-Translocator Complex Reveals a Role for Membrane Hairpins in Type III Secretion System Translocon Assembly(CELL PRESS, 2015-11-03) Nguyen, Van Sang; Jobichen, Chacko; Tan, Kang Wei; Tan, Yih Wan; Chan, Siew Leong; Ramesh, Karthik; Yuan, Yongming; Hong, Yunhan; Seetharaman, Jayaraman; Leung, Ka Yin; Sivaraman, J; Mok, Yu Keung; Assoc Prof Mok Yu Keung, Henry; BIOLOGY; CHEMISTRY; BIOLOGICAL SCIENCES© 2015 Elsevier Ltd. Type III secretion systems (T3SSs) are adopted by pathogenic bacteria for the transport of effector proteins into host cells through the translocon pore composed of major and minor translocator proteins. Both translocators require a dedicated chaperone for solubility. Despite tremendous efforts in the past, structural information regarding the chaperone-translocator complex and the topology of the translocon pore have remained elusive. Here, we report the crystal structure of the major translocator, AopB, from Aeromonas hydrophila AH-1 in complex with its chaperone, AcrH. Overall, the structure revealed unique interactions between the various interfaces of AopB and AcrH, with the N-terminal "molecular anchor" of AopB crossing into the "N-terminal arm" of AcrH. AopB adopts a novel fold, and its transmembrane regions form two pairs of helical hairpins. From these structural studies and associated cellular assays, we deduced the topology of the assembled T3SS translocon; both termini remain extracellular after membrane insertion.Publication Specificity and Inhibitory Mechanism of Andrographolide and Its Analogues as Antiasthma Agents on NF-kappa B p50(AMER CHEMICAL SOCIETY, 2015-02-01) Van, Sang Nguyen; Loh, Xin Yi; Wijaya, Hadhi; Wang, Jigang; Lin, Qingsong; Lam, Yulin; Wong, Wai-Shiu Fred; Mok, Yu Keung; Assoc Prof Mok Yu Keung, Henry; PHARMACOLOGY; BIOLOGY; PHYSIOLOGY; CANCER SCIENCE INSTITUTE OF SINGAPORE; CHEMISTRY; BIOLOGICAL SCIENCES© 2015 The American Chemical Society and American Society of Pharmacognosy. Andrographolide (1) is a diterpenoid lactone with an α,β-unsaturated lactone group that inhibits NF-κB DNA binding. Andrographolide reacts with the nucleophilic Cys62 of NF-κB p50 through a Michael addition at the Δ12(13) exocylic double bond to form a covalent adduct. Using computer docking, site-directed mutagenesis, and mass spectrometry, the noncovalent interactions between andrographolide and additional binding site residues other than Cys62 were found to be essential for the covalent incorporation of andrographolide. Furthermore, the addition reaction of andrographolide on Cys62 was highly dependent on the redox conditions and on the vicinity of nearby, positively charged Arg residues in the conserved RxxRxR motif. The reaction mechanisms of several of the analogues were determined, showing that 14-deoxy-11,12-didehydroandrographolide (8) reacts with NF-κB p50 via a novel mechanism distinct from andrographolide. The noncovalent interaction and redox environment of the binding site should be considered, in addition to the electrophilicity, when designing a covalent drug. Analogues similar in structure appear to use distinct reaction mechanisms and may have very different cytotoxicities, e.g., compound 6.Publication Homologous Lympho-Epithelial Kazal-type Inhibitor Domains Delay Blood Coagulation by Inhibiting Factor X and XI with Differential Specificity(CELL PRESS, 2018-07-12) Ramesh, Karthik; Lama, Dilraj; Tan, Kang Wei; Van, Sang Nguyen; Chew, Fook Tim; Verma, Chandra S; Mok, Yu Keung; Assoc Prof Mok Yu Keung, Henry; BIOLOGY; CHEMISTRY; BIOLOGICAL SCIENCES© 2018 Elsevier Ltd Despite being initially identified in the blood filtrate, LEKTI is a 15-domain Kazal-type inhibitor mostly known in the regulation of skin desquamation. In the current study, screening of serine proteases in blood coagulation cascade showed that LEKTI domain 4 has inhibitory activity toward only FXIa, whereas LEKTI domain 6 inhibits both FXIa and FXaB (bovine FXa). Nuclear magnetic resonance structural and dynamic experiments plus molecular dynamics simulation revealed that LEKTI domain 4 has enhanced backbone flexibility at the reactive-site loop. A model of the LEKTI-protease complex revealed that FXaB has a narrower S4 pocket compared with FXIa and hence prefers only small side-chain residues at the P4 position, such as Ala in LEKTI domain 6. Mutational studies combined with a molecular complex model suggest that both a more flexible reactive-site loop and a bulky residue at the P4 position make LEKTI domain 4 a weaker but highly selective inhibitor of FXIa. Ramesh et al. present the structure of the LEKTI domain 4 showing that it is a weak but specific inhibitor of Factor XIa. The bulkiness of the P4 residue and flexibility of the reactive-site loop determines the specificity.Publication Structural basis for the bacterial membrane insertion of dermcidin peptide, DCD-1L(NATURE PUBLISHING GROUP, 2017-10-24) Van, Sang Nguyen; Tan, Kang Wei; Ramesh, Karthik; Chew, Fook Tim; Mok, Yu Keung; Assoc Prof Mok Yu Keung, Henry; BIOLOGY; CHEMISTRY; BIOLOGICAL SCIENCES© 2017 The Author(s). Human dermcidin (DCD) is an antimicrobial peptide secreted constitutively by sweat glands. The anionic derivative, DCD-1L, comprises of the N-terminal 47 residues of DCD and one additional leucine residue. A previous NMR structure of DCD-1L in 50% TFE showed a partial helical conformation, and its crystal structure in the presence of Zn2+ outlined a hexameric linear α-helical bundle. Three different models to describe membrane insertion were proposed but no conclusion was drawn. In the current study, the NMR structure of DCD-1L in SDS micelles showed an "L-shaped" molecule with three fully formed α-helices connected by flexible turns. Formation of these helices in DCD-1L in the presence of POPG vesicles suggests that the acidic C-terminal region of DCD-1L can suppress the binding of DCD-1L to POPG vesicles at basic but not acidic pH. Mutation of charged residues on the N-terminal and C-terminal regions of DCD-1L cause differences in POPG binding, suggesting distinct functional roles for these two regions. Charged residues from these two regions are also found to differentially affect Zn2+ coordination and aggregation of DCD-1L in the absence or presence of SDS, as monitored by 1D NMR. Our data agrees with one of the three models proposed.