Figure 2: Time course of Cs⁺ accumulation in wild-type and sec22Δ-mutant yeast.

Whole-cell Cs⁺ concentrations were recorded at the indicated times after the addition of 50 μM CsCl. (a) Wild-type time course. (b) sec22Δ time course. Note the different scale of the concentration in (a) and (b). Continuous lines represent the fitting of the experimental data by the mathematical modelling and the predictions for the compartmentalization: the total intracellular (green), the cytoplasmic (blue) and the vacuolar (red) concentrations are depicted (Supplementary Methods). Steady-state Cs⁺ concentrations were predicted at 2.1 mM (wild type) or 1.1 mM (sec22Δ) in whole cells, at 0.40 mM (wild type) or 0.39 mM (sec22Δ) in the cytoplasm, and at 7.1 mM (wild type) or 3.2 mM (sec22Δ) in vacuoles. (c) Overlay of the initial time courses of wild type and sec22Δ. (a–c) Data are presented as means±s.d. (n=10). (d) Scheme of relative Cs⁺ compartmentalization in wild type and sec22Δ. Total Cs⁺ accumulation and vacuolar Cs⁺ compartmentalization were determined from whole cells after 18 h incubation in YPD supplemented with 50 μM radiolabelled Cs⁺. One complete pie represents 75 amol Cs⁺ per cell (complete uptake of wild-type cells). Total amount and vacuolar partitioning of Cs⁺ had been measured and were related to equal activity of the vacuolar marker α-mannosidase, resulting in a vacuolar Cs⁺ deposition of ~82% (wild type) and 55% (sec22Δ), respectively; n=3 independent vacuole isolations. (e) Scheme of relative Rb⁺ compartmentation measured in parallel to Cs⁺ distribution. The complete pie represents 81 amol per cell (wild type) and 87 amol per cell (sec22Δ). Rb⁺ deposition to the vacuole (70%) was identical for wild type and sec22Δ; n=3.