Unique ways to combat infections
13 Jan 2016 NUS scientists studied the regulatory system, recruiting machinery and translocation apparatus to deliver-virulence proteins into host cells to cause infection.
Pathogenic bacteria use an array of sophisticated machinery (‘secretion systems’) to channel various proteins (‘virulence factors’ or ‘effectors’) across the bacterial membrane and into the host cell. Once inside the host, these effectors will divert host cell function for the benefit and survival of the pathogen and thereby initiate infection. The inactivation or destabilisation of these secretion systems will prevent infection. There are seven known types of secretion systems (Types I-VII). The aim is to better understand and design novel ways to target the action(s) of three of these secretion systems (Types III, IV and VI) for therapeutic benefits.
Traditional antibiotics that cause cell death or inhibit bacterial growth invariably lead to drug resistance in bacterial strains when overused. This has led to the emergence of multi-drug resistant bacteria, which is a great public health concern.
A team led by Prof Jayaraman SIVARAMAN from the Department of Biological Sciences in NUS spent the past few years exploring and gaining insight into the detailed mechanism(s) of action of various bacterial secretion systems. The next major step in this research will be to identify and validate a group of small-molecule, drug-like compounds that can be used as anti-virulence drugs. From there, they hope to be able to combine these anti-virulence drugs with other antibiotic drugs to improve the efficiency of existing antibiotics in combating bacterial infection. By targeting the secretion system, they hope to develop broad-spectrum antibacterial compounds that pose no direct damage to the bacterium, and instead, prevent the delivery of effectors, thereby rendering the bacterium harmless. This approach would also facilitate the rapid clearance of the bacteria by the host immune system and potentially retain the resident beneficial microbe population in the host.
Their recent work has uncovered how the GrlR–GrlA protein complex regulates virulence in the Type Three Secretion System (T3SS) of Escherichia coli. They have also studied substrate recruitment in the Type Four Secretion System (T4SS), and found that plant tumours can be prevented by destabilising key recruiting proteins. In Type Six Secretion System (T6SS), they explored the structure-based function of EvpC and Hcp1, a major secreted protein that forms the molecular syringe apparatus of secretion, an essential component for the insertion of effectors into the host. This is also a marker protein for T6SS targeted for the production of monoclonal antibodies for diagnostic applications. These projects were conducted alongside several collaborators from NUS.
Left: Regulatory complex of the T3SS (positive and negative regulator complex; GrlR and GrlA complex). Middle: The role of key recruiting protein, VirD2 Binding Protein (VBP) in T4SS, on Agrobacterium tumefaciens tumorigenesis. Wounds inoculated with A. tumefaciens WT strain or GMV123 strain complemented with plasmid expressing VBP WT showed tumours; other wounds showed no tumour. This suggests that only A. tumefaciens harbouring the full-length VBP can induce tumour formation. Right: The translocon pore of T6SS. [Image credit: Jobichen CHACKO and J. Sivaraman].
1. Lim YT, Jobichen C, Wong J, Limmathurotsakul D, Li S, Chen Y, Raida M, Srinivasan N, MacAry PA, Sivaraman J, Gan YH. Extended loop region of Hcp1 is critical for the assembly and function of type VI secretion system in Burkholderia pseudomallei. Scientific Reports (2015) 5:8235.
2. Padavannil A, Jobichen C, Qinghua Y, Seetharaman J, Velazquez-Campoy A, Yang L, Pan SQ, Sivaraman J Dimerization of VirD2 binding protein is essential for Agrobacterium induced tumor formation in plants. PLoS Pathogens. (2014) 10(3):e1003948.
3. Padavannil A, Jobichen C, Mills E, Velazquez-Campoy A, Li M, Leung KY, Mok YK, Rosenshine I, Sivaraman J. Structure of GrlR-GrlA complex that prevents GrlA activation of virulence genes. Nature Communications (2013) 4:2546.
4. Jobichen C, Fernandis AZ, Velazquez-Campoy A, Leung KY, Mok YK, Wenk MR, Sivaraman J. Identification and characterization of the lipid-binding property of GrlR, a locus of enterocyte effacement regulator. Biochemical Journal (2009) 420(2): 191.
5. Jobichen C, Li M, Yerushalmi G, Tan YW, Mok YK, Rosenshine I, Leung KY, Sivaraman J. Structure of GrlR and the implication of its EDED motif in mediating the regulation of type III secretion system in EHEC. PLoS Pathogens (2007) 3(5):e69.