Figure 9.
Figure 9. Cytokinesis can progress with reduced or without PS at the septum edge. (A) The analysis of the ultrastructure of ingressing septa from WT cells (n = 130 septa) reveals two types of growing septum edges: rounded (54%) and sharp (46%). Many sharp septa present a delayed PS edge (arrows) uncoupled from the plasma membrane edge (arrowheads) and CR ingression. The recent complete septa (from 0.19 µm thickness) sometimes present a narrow medial region without a PS, indicating a previous sharp septum edge lacking a PS. Cells were grown in either YES or MM at 28°C and examined by TEM. Scale bars = 2 µm. (B) Magnifications of sharp septum edges in TEM images as in A. The sharp septum edges can be classified into three types according to a gradual increase in the sharp edge phenotype: type 1, overlapping PS and plasma membrane edges, the PS reaches the edge of the sharp septum; type 2, delayed PS edge with respect to the plasma membrane edge, leaving a space filled by a dark material similar to that of the SS; and type 3, advanced plasma membrane with no detectable PS material between both plasma membrane layers. The percentage and number of each type of sharp edge with respect to the total number of analyzed sharp edges (n = 60 sharp septum edges) are shown in parentheses (statistics include two images from Cortés et al., 2012). Images are from cells grown in different media as in A. Scale bars = 1 µm. (C) Quantification of rounded and sharp septum edges with respect to the growth medium of cells analyzed by TEM as in A (statistics include six images from Cortés et al., 2012, and two from Cortés et al., 2015). The number of each type of septum with respect to the total number of analyzed septa is shown in parentheses (YES, n = 67 septa; MM, n = 26 septa; MM+S, n = 37 septa). (D) Septa detected as complete by plasma membrane Bgs1 or syntaxin Psy1 localization present either complete or open PS (brackets) as detected by CW staining, similar to the complete septa observed by TEM in A. WT cells grown as in Fig. 1 D were analyzed by fluorescence microscopy as in Fig. 2 C. Scale bars = 2 µm. (E) Model of the ingression of both rounded and sharp septum edges. The addition of membrane vesicles allows the increase in plasma membrane surface and the decrease in CR tension (arrows). CR constriction coupled to septum plasma membrane and PS depositions permits overlapping plasma membrane and PS edges to make a rounded septum edge. CR constriction coupled to plasma membrane ingression but with reduced or delayed PS deposition generates a sharp septum edge, which can be classified into three types according to a gradual increase in the sharp edge phenotype, as described in B. PM, plasma membrane. Arrow, limit of PS edge; arrowhead, limit of septum membrane edge. Line points inside the septum membrane structure.

Cytokinesis can progress with reduced or without PS at the septum edge. (A) The analysis of the ultrastructure of ingressing septa from WT cells (n = 130 septa) reveals two types of growing septum edges: rounded (54%) and sharp (46%). Many sharp septa present a delayed PS edge (arrows) uncoupled from the plasma membrane edge (arrowheads) and CR ingression. The recent complete septa (from 0.19 µm thickness) sometimes present a narrow medial region without a PS, indicating a previous sharp septum edge lacking a PS. Cells were grown in either YES or MM at 28°C and examined by TEM. Scale bars = 2 µm. (B) Magnifications of sharp septum edges in TEM images as in A. The sharp septum edges can be classified into three types according to a gradual increase in the sharp edge phenotype: type 1, overlapping PS and plasma membrane edges, the PS reaches the edge of the sharp septum; type 2, delayed PS edge with respect to the plasma membrane edge, leaving a space filled by a dark material similar to that of the SS; and type 3, advanced plasma membrane with no detectable PS material between both plasma membrane layers. The percentage and number of each type of sharp edge with respect to the total number of analyzed sharp edges (n = 60 sharp septum edges) are shown in parentheses (statistics include two images from Cortés et al., 2012). Images are from cells grown in different media as in A. Scale bars = 1 µm. (C) Quantification of rounded and sharp septum edges with respect to the growth medium of cells analyzed by TEM as in A (statistics include six images from Cortés et al., 2012, and two from Cortés et al., 2015). The number of each type of septum with respect to the total number of analyzed septa is shown in parentheses (YES, n = 67 septa; MM, n = 26 septa; MM+S, n = 37 septa). (D) Septa detected as complete by plasma membrane Bgs1 or syntaxin Psy1 localization present either complete or open PS (brackets) as detected by CW staining, similar to the complete septa observed by TEM in A. WT cells grown as in Fig. 1 D were analyzed by fluorescence microscopy as in Fig. 2 C. Scale bars = 2 µm. (E) Model of the ingression of both rounded and sharp septum edges. The addition of membrane vesicles allows the increase in plasma membrane surface and the decrease in CR tension (arrows). CR constriction coupled to septum plasma membrane and PS depositions permits overlapping plasma membrane and PS edges to make a rounded septum edge. CR constriction coupled to plasma membrane ingression but with reduced or delayed PS deposition generates a sharp septum edge, which can be classified into three types according to a gradual increase in the sharp edge phenotype, as described in B. PM, plasma membrane. Arrow, limit of PS edge; arrowhead, limit of septum membrane edge. Line points inside the septum membrane structure.

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