ER arrival sites for COPI vesicles localize to hotspots of membrane trafficking (2016)
Figure 2

Figure 2. Localization patterns of COPI•Dsl BiFC foci during growth and starvation

A. Dsl1pVN•ε-COPVC BiFC signal localization pattern throughout the budding cycle. Time-lapse images of agarose-embedded cells were recorded at RT with continuous PM Glc+ura medium supply. Arrowheads mark positions of BiFC foci. Fluorescence signals exhibited a characteristic and reproducible localization pattern, which favored sites of high material turnover during budding. Scale bar 10 µm.

B. Dsl1pVN•ε-COPVC BiFC foci showed different subcellular distribution depending on the growth status of cells. Cells were cultured in PM+ura medium with or without 2 % glucose at 30 °C for 90 min, and imaged. BiFC signals dispersed under glucose starvation, resulting in an ER exit site-like fluorescence pattern. Scale bar 10 µm.

C. The effect of carbon source depletion on COPI•Dsl BiFC signal polarization. Cells carrying Dsl1pVN•ε-COPVC were starved in glucose-free PM+ura medium for 90 min. Then the medium was replaced by PM Glc+ura with 2 % glucose, while imaging the cells continuously at 1 min intervals. Two exemplary cells are shown (upper and lower panel). Starvation-induced dispersed BiFC signals reversibly returned to polarized distribution within minutes after addition of glucose-containing medium. Arrowheads mark the point of signal repolarization, arrows indicate resumed bud outgrowth. Scale bar 5 µm.

D. βʼ-COPVN•Dsl3pVC BiFC signals during mating. Fluorescence micrographs of the BiFC pair β’-COPVN•Dsl3pVC (green) are superimposed with the DIC image (grey). Two haploid strains carrying βʼ-COPVN•Dsl3pVC were allowed to mate in YEPD liquid culture for 2 h at 30 °C. Then time-lapse recordings were made in agarose embedding at RT with continuous PM Glc+ura medium supply. Arrowheads mark positions of BiFC foci. Fluorescence signals exhibited a characteristic and reproducible localization at sites of high material turnover during zygote formation. Scale bar 10 µm.

E. Comparison of the BiFC localization pattern in COPI•Dsl and COPI•COPI strains. Time-lapse images of agarose-embedded cells carrying the COPI•Dsl BiFC pair βʼ-COPVN•Dsl3pVC or the COPI•COPI BiFC pair βʼ-COPVN•α-COPVC were recorded at RT with continuous PM Glc+ura medium supply at 5 min intervals. Single cell pseudokymographs were generated by concatenating the central section of the cell (sliced along the pole axis, frames at double the width of the bud neck) of a time-lapse dataset. The COPI•Dsl strain showed one single dominant spot at the bud tip and later the bud neck. In contrast, the COPI•COPI strain additionally exhibited multiple spots with no apparent polarization tendency, which were localized mainly in the mother cell. Schematic yeast cell representations depict the typical BiFC foci localization pattern of the strains. Scale bar 10 µm.

F. Fluorescence recovery after photobleaching of BiFC signals. Time-lapse images of agarose-embedded cells carrying the β’-COPVN•Dsl3pVC BiFC pair were recorded at RT with PM Glc+ura medium supply at 2 min intervals. Fluorescence of ROIs marked in dashed lines (where the full cell, full bud, or only the bud foci were bleached) were depleted by three repetitions of fluorescence excitation at 177 µs/pixel and 100 % laser intensity. Arrowheads: reappearing fluorescence signal; scale bar 2 µm.

G. Densitometric measurements of fluorescence intensities in photobleached cells. Representative data sets from five independent experiments are displayed, featuring cells that had been bleached entirely (black lines), at the bud only (dark blue lines), or at the fluorescent foci at the bud tip only (light blue line). Values are displayed in percentage of initial fluorescence, and were corrected for photobleaching effects by normalization against fluorescence of unbleached cells. Fluorescence recovery was typically not continued to full recovery stage because of the photobleaching artefacts occurring during long-term imaging of the samples.