Indeed, the response to unfolded protein stress GO term was significantly
repressed upon melittin treatment (Additional File 4). HSC82 was repressed by PAF26, and the corresponding deletion strain was selectively more resistant to PAF26 (Figure 5C). Interaction of PAF26 with S. JQEZ5 cerevisiae cells We have previously reported selleck kinase inhibitor that PAF26 is capable to interact with and be internalized by the hyphal cells of the filamentous fungus P. digitatum at sub-inhibitory concentrations (0.3 μM) . PAF26 is markedly less active against S. cerevisiae than towards P. digitatum  and, accordingly, although internalization of fluorescently labeled PAF26 into S. cerevisiae FY1679 could be demonstrated through confocal Selleck PI3K inhibitor microscopy, 100-fold higher peptide concentrations (30 μM) were required (Figure 6A). Figure 6
Fluorescence microscopy of S. cerevisiae exposed to FITC-PAF26. (A) Internalization of FITC-PAF26 into S. cerevisiae FY1679 demonstrated by confocal fluorescence microscopy. Cells were exposed to 30 μM FITC-PAF26 for 30 min. Bright-field (A1) and fluorescence (A2) micrographs of the same field are shown. (B) Interaction of FITC-PAF26 with S. cerevisiae BY4741 visualized by fluorescence microscopy: DIC bright field image, as well as FITC, propidium iodide (PI), and calcofluor white (CFW) signals of the same field are shown. Cells were incubated with 30 μM FITC-PAF26 at 30°C for 2 h, and then at 20°C with 2 μM PI and 25 μM CFW for 5 min. Open arrowheads
indicate peptide internalization (compare location of the CW outer signal of CFW with the internal signal of PI and the FITC fluorescence resulting from FITC-PAF26). Solid arrowhead indicates the lower FITC signal in the vacuole compared to the cytosol. In order to determine whether the sensitivity to PAF26 is correlated with the interaction and uptake of the peptide into S. cerevisiae, and also how this is associated with cell viability, we set up an assay MG132 in which cells were treated with FITC-PAF26 followed by treatment with the cell death marker propidium iodide (PI) and the CW stain CFW (Figure 6B). Approximately 5-20% of S. cerevisiae BY4741 were labeled by FITC-PAF26 under these assay conditions (see also below), and such labeling co-localized with that of PI. Also, staining by CFW showed strong cell wall disorganization for those non-viable cells into which peptide were located. Despite not using confocal optics as in Figure 6A, this three-fluorophore staining also supports the internalization of the peptide and confirmed that cells showing the highest peptide signal were the most permeable to PI. Our microscopy experiments also show FITC-PAF26 accumulation in the cytosol, excluded from the vacuole (Figures 6A and 6B). Selected deletion mutants were analyzed using this approach (Figure 7, high magnification and data on CFW staining are not shown for simplicity).