A robust protocol for the isolation of cellular proteins from Xanthomonas campestris to analyze the methionine effect in 2D-gel experiments.

Two-dimensional PAGE (2D-PAGE) is a key technique for the separation of complex protein samples to survey the protein inventory of prokaryotic microorganisms. Although the preparation of proteins is a critical step in 2D gel experiments, its effect on the outcome of proteome data is often underestimated. In this work, we show that the choice of protein isolation method can have a profound impact on the quality of 2D gels of protein extracts from prokaryotes. Based on Xanthomonas campestris, a commercially relevant producer of the thickening agent xanthan, we validated a phenol extraction protocol for the purification of bacterial proteins that provides excellent 2D gel separation. As a proof of concept, this method was used to study the effect of methionine-a medium compound that reduces the xanthan output of industrial fermentations-on the cellular proteome of Xanthomonas. The detection of nine regulated proteins associated with sulfur metabolism (Cgl, CysI, CysJ, CysK, MetH1, MetY) and sugar nucleotide biosynthesis (Pgi, Ugd, XanA) proved the efficiency of phenol extraction for the screening of statistically significant abundance changes in 2D gel spots and MALDI-TOF-MS based identification in bacteria. Since this method is very robust, it may be useful for the study of other prokaryotes that are relevant in industrial biotechnology.