Interaction of Chs4p and Chs3p mediates Chitin synthase III activation in Saccharomyces cerevisiae.
Abigail Reyes, Maria Sanz, Rosario Valle, Angel Duran, Cesar Roncero
Microbiologia y Genetica, CSIC/Universidad de Salamanca, Avda. Campo Charro, Salamanca, 37007, Salamanca, Spain
Chitin synthesis in S.cervisiae depends mostly on the CSIII activity. CSIII is regulated at the postranslational level through Chs4p and Shc1p, two homologue proteins developmentally regulated. Both proteins interact with Chs3p activating the CSIII, Chs4p is rapidly degraded during sporulation leaving CSIII activation to Shc1p, which is induced during the process. However, CSIII regulation does not depend on Chs4p levels since they are constant during the different stages of cell division and Chs4p degradation is not promoted after cell cycle blockage. Therefore, the interaction of Chs4p and Chs3p appears to be the factor triggering CSIII activation. Chs4p interacts with Bni4p and Chs3p but interaction of Chs4p and Chs3p is sufficient to promote CSIII activation thus separating the two function of Chs4p. However, Chs4p interacts with both proteins through the same protein domain. Chs4p is a membrane-associated protein transported through the secretion machinery. Sub-cellular fractionations indicate that Chs4p association with membranes depends on his own signals but also on the interaction with Chs3p and Bni4p, cell wall associated proteins by themselves. The most distal C-terminal region of Chs4p is not required for function since it is dispensable for his interaction with Bni4p or Chs3p. We have obtained Chs4p mutants showing normal interaction patterns but failing in the activation of CSIII. These results will be discussed in the context of CSIII activation.