Fission yeast GPI inositol deacylase Bst1 regulates ER-Golgi transport and functions in late stages of cytokinesis.

The Munc13/UNC-13 family protein Ync13 is essential for septum integrity and cytokinesis in fission yeast. To further explore the mechanism of Ync13 functions, spontaneous suppressors of mutants, which can suppress the colony-formation defects and lysis phenotype of mutant cells, are isolated and characterized. One of the suppressor mutants, -, shows defects in the cytokinetic contractile ring constriction, septation, and daughter cell separation, similar to mutant. Bst1, a predicted GPI inositol deacylase, was an uncharacterized protein in fission yeast. It localizes to ER and puncta structures in the cytoplasm. The Bst1 puncta overlaps frequently with Anp1, which is a marker of endoplasmic reticulum (ER)-Golgi transport, but rarely with trans-Golgi marker Sec72. The nuclear ER signal of Anp1 increases in mutant, whereas Sec72 localization shows no obvious changes. In addition, more cytoplasmic puncta structures of COPII subunits, Sec13 and Sec24, are observed in mutant, and acid phosphatase secretion is compromised without Bst1. Consistently, the division site targeting of the β-glucanase Eng1 and α-glucanase Agn1 is reduced in and mutant. Taken together, our results suggest that Bst1 regulates ER-Golgi transport and is involved in cytokinesis through regulating the secretion of glucanases.