Scientists based at the University of Venda and the University of Zululand investigated the effect of citrate coated gold nanoparticles on the integrity of E. coli cells. Studies conducted by the scientists demonstrated that synthetic gold nanoparticles coated by either citrate or cysteine readily associated with protein. In addition, the nanoparticles were capable of suppressing the heat-induced aggregation of model proteins such as citrate synthase and malate dehydrogenase in vitro. In this respect gold nanoparticles mimic the functions of the cell’s molecular chaperone system. However, at very high concentrations the nanoparticles agglomerated and caused protein aggregation. Chaperones are molecules that prevent protein misfolding, refold misfolded proteins and/or suppress protein aggregation. E. coli Hsp70 (DnaK) is one of the most prominent molecular chaperones. Because gold nanoparticles suppress protein aggregation, they share this feature with DnaK in vitro. In the current study, E. coli cells deficient for DnaK (DnaK^-) were exposed to variable levels of citrate-coated gold nanoparticles. This was followed by investigating the effect of the nanoparticles on the morphology of the cells using TEM analysis. The scientists further investigated the effect of the gold nanoparticles on the proteomic integrity of the E. coli cells. Parallel studies were conducted on E. coli cells in which DnaK function was restored (DnaK⁺). The cells were incubated at their ambient growth temperature (30°C) and another batch was exposed to a non-permissive temperature of 40°C. Based on transmission electron microscopic (TEM) images, it was observed that the gold nanoparticles induced toxicity on the E. coli DnaK- cells and that the toxicity was alleviated in cells in which DnaK was heterologously re-introduced. The cells internalised the gold nanoparticles leading to morphological changes such as exaggerated filamentation, complexing of the cytoplasmic material with the nanoparticles, and membrane pooling off the cell wall. This led to the death of E. coli DnaK^- cells. However, E. coli DnaK⁺ cells were less susceptible to the toxicity of the nanoparticles. Notably, E. coli DnaK⁺ cells exposed to toxic levels of the nanoparticles exhibited distinct zones in the cytosol which appeared to restrict the gold nanoparticles to distinct sites in the cell. It appears that DnaK directly or indirectly alleviates toxicity of the nanoparticles by restricting their localisation to distinct cellular sites.

It was further noted that the E. coli DnaK^- cells incubated at 30°C in the absence of gold nanoparticles exhibited a proteomic solubility profile that resembled that of cells that were exposed to gold nanoparticles. In addition, E. coli DnaK^- cells exposed to gold nanoparticles over-expressed GroEL, a molecular chaperone that is known to facilitate protein folding. Since GroEL was not over-expressed by E. coli DnaK⁺ cells, it must have been upregulated to compensate for lack of DnaK function towards mitigating nanoparticle induced toxicity. On the other hand, the E. coli DnaK⁺ cells cultured in the presence or absence of the gold nanoparticles exhibited similar protein solubility profiles. Thus DnaK suppressed the aggregation of the proteomic constituents of the E. coli DnaK⁺ cells exposed to nanoparticles even upon elevating the incubation temperature to 40°C. Altogether, besides restricting the nanoparticles to distinct cellular sites, it appears that DnaK relieves the toxicity of gold nanoparticles in E. coli through its chaperone function. Below is a summary of the study and the model illustrating the proposed function of DnaK.

Authors:

Stanely Makumire is a PhD candidate, Department of Biochemistry, University of Venda	Addmore Shonhai (PhD) is Associate Professor and Head of Biochemistry Department, University of Venda	Prof Neerish Revaprasadu is the SARCHI Chair of Nanotechnology at University of Zululand

Journal reference: Makumire S, Revaprasadu N, Shonhai A (2015) DnaK protein alleviates toxicity induced by citrate-coated gold nanoparticles in Escherichia coli. PLoS ONE 10(4): e0121243. doi:10.1371/journal.pone.0121243