The multiprotein exocyst complex is essential for cell separation in Schizosaccharomyces pombe

Little is known about how fission yeast cells separate after division. While the actomyosin ring and division septum have been studied, the mechanism of cell separation remains unclear. Prior research has shown that S. pombe divides through medial fission. A division septum forms alongside the contractile ring. Cleavage of the inner septum is required for daughter cell release. No prior work had resolved how this cleavage is targeted or executed. This gap motivated the investigation of exocyst function in cell separation. The exocyst complex is known to mediate exocytosis in other systems. That uncertainty drove the analysis of exocyst mutants in S. pombe.

This study aimed to determine the role of the exocyst complex in cell separation in S. pombe. The researchers focused on a mutant defective in cell separation but not in other cytokinesis steps. They sought to identify if exocyst function is essential for cleavage of the division septum. The study also examined how exocyst components localize during cell growth and division. The goal was to clarify whether exocytosis is required for cell separation. The researchers tested if exocyst mutants could still assemble division septa. They examined whether exocyst function is necessary for viability. The study aimed to propose a model for exocyst involvement in septum cleavage.

The researchers used a sec8-1 mutant to study cell separation defects. They observed vesicle accumulation and reduced acid phosphatase secretion. Biochemical assays confirmed physical interactions between exocyst components. Immunofluorescence tracked the localization of exocyst proteins during cell growth. They examined localization at interphase cell ends and during cytokinesis. F-actin dependence of exocyst localization was tested using disruption methods. The team analyzed multiple mutations in exocyst components. They assessed viability and cell separation in each mutant strain.

sec8-1 mutants failed to separate daughter cells after division. These mutants accumulated 100-nm vesicles and showed reduced acid phosphatase secretion. Sec8p physically interacts with Sec6p, Sec10p, and Exo70p. Exocyst proteins localized to sites of active exocytosis in interphase cells. They also localized to the medial region during cytokinesis. This localization was F-actin dependent but not exocytosis dependent. Mutations in exocyst components caused cell viability defects. All mutants could elongate and assemble division septa but failed to separate.

The exocyst complex is essential for cell separation in S. pombe. The researchers propose it targets enzymes that cleave the division septum. Exocyst function is not required for septum assembly or cell elongation. Minimal exocyst activity allows these processes to proceed. The study supports a model where exocyst directs cleavage enzymes to the septum. The findings suggest that exocytosis is not directly required for cell separation. The exocyst may deliver enzymes to the septum rather than mediate general secretion. The results highlight the exocyst’s role in spatial regulation of cell division.