Ags1 is essential for the primary septum strength needed to support the cell separation internal pressure This in turn gave rise to a new three-layered septum structure with a twisted primary septum flanked by remedial secondary septa ( Fig. The primary septum could not drill the cell wall, but instead the base appeared anchored to the new ICW, which extended along the naked primary septum. At longer repression times, a new remedial internal cell wall (ICW) layer appeared ( Figs. Sorbitol delayed the lytic phenotype, revealing a primary septum synthesized with no secondary septum ( Fig. In addition, Ags1 is also essential for primary septum anchorage and MTD assembly. This indicates that Ags1 is essential for a straight primary septum synthesis, and suggests an important Ags1 cooperation with Bgs1 and the CAR. 7, B–D) and not anchored primary septa (arrow) and disorganized MTD structures ( Fig. The Ags1-depleted cells presented strong septum defects of abruptly twisted ( Fig. In agreement with previous data from ags1 + point mutants ( Hochstenbach et al., 1998 Katayama et al., 1999), the Ags1-depleted cells showed a considerable reduction (53%) in the cell wall α-glucan content ( Table 1). A similar lysis phenotype was observed in the mok1-664 mutant at a restrictive temperature ( Fig. Total lysis reached 70% of the cells ( n = 420) after 7 h of repression, and sorbitol reduced it to 7% ( n = 986 Fig. 5 B, arrowhead) and mostly from the septum ( Fig. Morphological observations revealed that Ags1 depletion promotes cell lysis and cytoplasm release from the lateral region of the poles ( Fig. Cell growth arrested after 4 h of ags1 + repression and osmotic stabilization protected the cells, delaying the growth arrest to at least 12 h ( Fig. To study the lethal effect of the absence of Ags1, a strain expressing a single integrated ags1 + copy under the control of the thiamine-repressible 81X- nmt1 + promoter was analyzed (see Materials and methods). However, their importance for cell wall and septum structure and function is still unknown ( Ribas et al., 1991 Hochstenbach et al., 1998 Katayama et al., 1999). α(1-3) and branched β(1-3)glucans are essential for cell shape maintenance. In addition to β-glucans, other polysaccharides, like the α(1-3)glucan, may be present in the primary septum as well. L-BG is necessary but not sufficient for primary septum formation and is the polysaccharide that specifically interacts with the fluorochrome Calcofluor white (CW) in S. Immunoelectron microscopy (IEM) studies delimited the branched β(1-6)glucan in the cell wall and secondary septum, the branched β(1-3)glucan in the cell wall and both primary and secondary septum, and a linear β(1-3)glucan (L-BG) mainly present in the primary septum and a small amount in the cell wall ( Humbel et al., 2001 Cortés et al., 2007). The fission yeast cell wall consists of an outer layer rich in galactomannoproteins and an inner layer comprised of β(1-3), β(1-6), and α(1-3)glucans ( Pérez and Ribas, 2004 Grün et al., 2005). Consequently, the absence of Ags1 and therefore α(1-3)glucan generates a special and unique side-explosive cell separation due to an instantaneous primary septum tearing caused by the turgor pressure. We show that α(1-3)glucan is essential for both secondary septum formation and the primary septum structural strength needed to support the physical forces of the cell turgor pressure during cell separation. Moreover, we define the essential functions of α(1-3)glucan in septation and cell separation. Here, we generate a physiological Ags1-GFP variant and demonstrate a tight colocalization with Bgs1, suggesting a cooperation in the important early steps of septum construction. Fission yeast cell wall α(1-3)glucan is essential, but nothing is known about its localization and function in the cell wall or about cooperation between the α- and β(1-3)glucan synthases Ags1 and Bgs for cell wall and septum assembly. In yeast, the septum has to be selectively digested during the critical cell separation process. Fungal cytokinesis requires the assembly of a dividing septum wall.
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