Bgs4 is required for PS completion but not for general septum closure. (A) Absence of Bgs4 promotes a septation increase in live but not in total cells (left). Bgs4 absence produces an increase in CW-stained open septa and concomitant decrease in complete septa (right). Arrow shows start of septum defects and cell lysis. Cells were grown in MM with thiamine (+T; repressed) and 1.2 M sorbitol (S). (B) Bgs4 absence causes a defect in CW-stained but not in GFP-Bgs1 septum completion (arrowheads). Cells were grown in MM+S+T for 10 h. (C) Bgs4 absence generates an increase in CW-stained open PS (arrow), not detected in GFP-Bgs1 septa (left). Defective open PS are detected in both advanced (with respect to the CW signal) and complete GFP-Bgs1 septa (right). Cells were grown as in A. Error bars indicate SD. (D) The defect in open PS does not correspond to a general defect in septum synthesis or CAR contraction. Progression (bracket) of WT PS coincides with that of septum membrane (Bgs1, Bgs3, and Psy1) and CAR (Rlc1) proteins but not in the absence of Bgs4, in which the CAR stays attached to the PM, whereas the PS formation is uncoupled and delayed. Cells were grown as in B. The number of experiments and cells or septa analyzed is shown in each case. Bars: (cells) 5 µm; (septum details) 1 µm.