Characterisation of the Saccharomyces cerevisiae cell separation machinery: Sdm1 and its possible role in cell separation

Abstract

The final act of the cell cycle in Saccharomyces cerevisiae is cell separation. Inactivation of the transcription factor encoding gene, ACE2, leads to a defect in cell separation. Ace2 plays a central role in cell separation by regulating daughter cell specific expression of endochitinase (CTS1) and at least 3 putative glucanase encoding genes, DSE2, DSE4 (ENG1) and SCW11. The products of these genes degrade the tri-laminar septum that holds mother and daughter cells together. ACE2, itself, is regulated by the RAM (Regulation of Ace2 activity and cellular Morphogenesis) network; inactivation of RAM network proteins results in defects in cell separation and mis-localisation of Ace2. To define the components of the cell separation machinery in S. cerevisiae, a screen for mutants that fail to separate was undertaken. A total of 178 novel cell separation mutants were identified; 11 were uncharacterised genes implicated in cell separation, and of these, 6 were implicated in cell wall integrity, while one was implicated in the MAP kinase pathway. Furthermore, ubquitination and glycosylation were implicated to play a role in cell separation. The uncharacterised gene, YIR016W, termed Defective in Separation of Daughter and Mother Cell 1 (SDM1), was identified to play an important role in cell separation. A yeast-2-hybrid screen identified 15 novel protein-protein interactions not previously described for Sdm1. The uncharacterised gene, YOL036W, was identified to interact with Sdm1, and the RAM proteins, Mob2 and Cbk1, in a yeast-2-hybrid screen. Furthermore, Sdm1 and Yol036w are paralogues. We propose that Sdm1 and Yol036w are involved in ER to golgi trafficking. Data presented here shows that Ace2 appears to be mis-localised in sdm1 cells; we also show that there may be a potential budding pattern defect in sdm1 cells. We propose that Sdm1 is involved in the regulation of Ace2 localisation to the daughter cell nucleus at the end of M/G1, and that Sdm1 is a negative regulator of cell cycle progression at G1/M.