Elucidating the molecular mechanism of the inhibitory effect of epigallocatechin-3-gallate on Microcystis aeruginosa

Abstract

Epigallocatechin-3-gallate (EGCG), an eco-friendly polyphenol, has a strong inhibitory effect on the bloom-forming cyanobacterium Microcystis aeruginosa. In order to reveal the molecular mechanism of algal inhibition of plant polyphenols, quantitative proteome analysis based on iTRAQ (isobaric tags for relative and absolute quantification) was applied to investigate EGCG-induced proteomic changes in M. aeruginosa. Following treatment with EGCG for 48 h, the total protein content was compared with that of untreated cells, and 88 differentially expressed proteins were identified, of which 30 were upregulated and 58 were downregulated. Proteins involved in chlorophyll biosynthesis, carbon and nitrate assimilation and cell division were among the most downregulated, which resulted in growth suppression. By contrast, anti-oxidative proteins and molecular chaperones, such as superoxide dismutase, glutaredoxin, and heat shock proteins, were distinctly upregulated. Eighteen potentially crucial proteins were selected for assessment of transcription by real-time quantitative PCR, which confirmed the results of proteomic experiment. In addition, immunoblotting confirmed downregulation of three representative proteins, magnesium-protoporphyrin IX monomethyl ester [oxidative] cyclase AcsF, glutamine synthetase GlnN, and metalloprotease FtsH, following EGCG treatment, consistent with the results of iTRAQ quantitation experiment. It is speculated that chlorophyll biosynthesis, carbon and nitrate assimilation, and cell division are the main inhibition targets of EGCG.