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Saccharomyces cerevisiae Bni4p directs the formation of the chitin ring and also participates in the correct assembly of the septum structure

Instituto de Microbiología Bioquímica, Consejo Superior de Investigaciones Científicas/Universidad de Salamanca and Departamento de Microbiología y Genética, Universidad de Salamanca. 37007 Salamanca, Spain

Correspondence
César Roncero
crm{at}usal.es

In Saccharomyces cerevisiae cytokinesis is efficiently achieved when a concerted series of events take place at the neck region, leading to septum formation. Here it is shown that Bni4p plays a crucial role in this process. bni4 mutants contain normal amounts of chitin and show normal chitin synthase III (CSIII) activity, but are partially resistant to Calcofluor White (CFW), probably due to the striking pattern of chitin distribution. CFW vital staining shows that chitin is synthesized in daughter cells and that it is also asymmetrically deposited at the mother-side of the neck in large-budded cells. This specific pattern coincides with that of Chs4p and Chs3p proteins. Alternatively, staining of unbudded cultures confirmed that Bni4p directs early chitin ring assembly, but is no longer required for the chitin deposition that occurs late in the cell cycle at cytokinesis. Consequently, this work provides a strategy to genetically discriminate between the absence of chitin synthesis (chs3 mutant) and failure in chitin ring assembly (bni4 mutants). The characterization of double mutants affected in chitin synthesis and primary septum (PS) assembly (myo1 and chs2) provides evidence for the cooperation of Bni4p in PS formation besides its role in chitin ring assembly. In addition, it is shown that the chitin ring, but not the late deposition of chitin, cooperates in the correct assembly of the actomyosin ring and the PS when the biological function of the septins is compromised. We conclude that Bni4p is not only required for the assembly of the chitin ring, but is also involved in septum architecture and the maintenance of neck integrity.

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