Bgs4 is essential to protect the cell integrity from an excess of cell wall degradation at the start of cell separation. (A–D) Bgs4 absence causes cell lysis and release of cytoplasm after septum maturation from either one or both sister cells. (A) The lysis in one cell (left) appears earlier, later increasing in both cells (right). Cells were grown as in Fig. 1 B. (B) Percentage of each cell lysis. Error bars indicate SD. (C) Model of differential cell lysis in one or both sister cells. The extension of asymmetric cell wall weakness caused by asymmetric cell growth (red arrows) and the septum position determine lysis (arrowheads) in one or both cells. (D) Cell lysis and cytoplasm release (arrow) from either one (left) or both (right) cells. Cells were grown as in Fig. 1 B and imaged by time-lapse. (E and F) TEM details of cell separation. (E) Controlled WT cell wall DR degradation and gradual PS degradation (left). Bgs4 absence causes an excess of cell wall degradation, leaving the PM exposed to the medium (right). (F) Cell lysis and cytoplasm release (arrow) at the start of cell separation, before PS degradation is detected. Cells were grown as in D. F, fuscannel; FS, fission scar. Bars: (cells) 5 µm; (septum details) 1 µm.