Role of Septins and the Exocyst Complex in the Function of Hydrolytic Enzymes Responsible for Fission Yeast Cell Separation

Abstract

Cell separation in Schizosaccharomyces pombe is achieved by the concerted action of the Eng1 endo-␤-1,3-glucanase and the Agn1 endo-␣-1,3-glucanase, which are transported to the septum and localize to a ringlike structure that surrounds the septum. The requirements for the correct localization of both hydrolases as a ring were analyzed using green fluorescent protein fusion proteins. Targeting to the septum required a functional exocyst, because both proteins failed to localize correctly in sec8-1 or exo70⌬ mutants, suggesting that Agn1 and Eng1 might be two of the cargo proteins present in the vesicles that accumulate in exocyst mutants. Septins and Mid2 were also required for correct formation of a ring. In their absence, Eng1 and Agn1 were found in a disklike structure that spanned the septum, rather than in a ring. Even though septin and mid2⌬ mutants have a cell separation defect, the septum and the distribution of linear ␤-1,3-glucans were normal in these cells, suggesting that mislocalization of Eng1 and Agn1 might be the reason underlying the failure to separate efficiently. Thus, one of the functions of the septin ring would be to act as a positional marker for the localization of hydrolytic proteins to the medial region.

Cooperation Between the Septins and the Actomyosin Ring and Role of a Cell-Integrity Pathway During Cell Division in Fission Yeast

“…In the absence of the septins, the septum that forms has defects, the localization of lytic enzymes that function in cell separation is perturbed (see Martín-Cuadrado et al 2005;Sipiczki 2007), or both. If the cell-wall-integrity pathway is functioning normally, the problems can be recognized and repaired, and the cells usually divide successfully (albeit typically after a significant delay).…”

“…Surprisingly, mutants lacking the septins are viable and form seemingly complete septa with approximately normal timing. These mutants do, however, display a variable delay in separation of the daughter cells, suggesting that the septins play some role(s) in the proper completion of the septum or in subsequent processes necessary for cell separation (Longtine et al 1996;An et al 2004;Martín-Cuadrado et al 2005).…”

Mechanisms of cytokinesis in budding yeast

“…Similarly, septins are required for cytokinesis in some animal cells [Neufeld and Rubin, 1994;Fares et al, 1995;Kinoshita et al, 1997;Estey et al, 2010]. In contrast, septins are not required for cell viability but are required for efficient cell separation in the fission yeast S. pombe [Martin-Cuadrado et al, 2005;Wu et al, 2010].…”

The Actomyosin Ring Recruits Early Secretory Compartments to the Division Site in Fission Yeast

“…Application of the drug brefeldin A (BFA), which inhibits the exchange factor for the small GTPase Arf, renders fission yeast cells incapable of localizing the (1,3)-␤-glucan-synthase catalytic subunit Cps1p to the division site, presumably because of a block in secretion . Furthermore, the multiprotein complex involved in the late steps of the exocytic pathway named exocyst is essential for the proper localization of the endo-␤-1,3-glucanase, Eng1p (Martin-Cuadrado et al, 2005). In fission yeast, the exocyst-mediated secretion is restricted to the cellular growth regions including cell tips and the cell division site In exocytosis, vesicles originating from the secretory pathway or endosomal recycling (Engstler et al, 2004) fuse with the plasma membrane.…”