Q-MT bundle formation does not depend on GTP depletion but rather involves the SPB. (A) GTP concentration decreases upon quiescence entry. (B) In proliferating cells, a MPA treatment (100 µg/ml) causes a massive GTP concentration drop. Proliferating cells were incubated 30 min or 2 h with either MPA, MPA + guanine (0.3 mM) to reverse the MPA effect, or guanine alone as a control. In A and B, N = 4 experiments with two samples per experiment. (C) MPA-induced GTP droop does not affect MT dynamics in proliferating cells. MT growth and shrinkage speed were determined in proliferating WT cells expressing GFP-Atb2 treated (2 h) or not with MPA. (D and E) MT stabilization involves the SPB. (D) At various times after carbon exhaustion (left, red dashed line), cell expressing GFP-Atb2 (green) and Sfi1-CFP (red) were treated for 10 min or 8 h with benomyl and then imaged. Numbers indicate the mean number of MT bundle per cell (N = 2 experiments and n > 200 cells for each time point). (E) WT cells were grown 6 h after carbon starvation and incubated with benomyl for the indicated times (left). A cartoon (middle) representing either a MT bundle random stabilization model or model biased toward the stabilization of the SPB-associated MT bundle. Red, SPB; blue, nuclear membrane; green, MT bundle. Cells displaying multiple MT bundles (red bars) or only a single MT bundle associated with the SPB (green bars) or not (gray bars) were scored for cells incubated with benomyl for the indicated times. N = 3 experiments and n > 200 cells per time point. Error bars are SD. The percentage of cells displaying a unique MT bundle associated with the SPB calculated theoretically using a random stabilization prediction (black asterisks) or SPB-biased stabilization (blue askterisks) are indicated (see Materials and methods for details). For all graphs, means and SD are indicated. Bars, 2 µm.