Hyperpolarized growth of Saccharomyces cerevisiae cak1P212S and cla4 mutants weakens cell walls and renders cells dependent on chitin synthase 3.

In a screen for cell wall defects in Saccharomyces cerevisiae, we isolated a strain carrying a mutation in the Cdc28-activating kinase CAK1. The cak1P212S mutant cells exhibit multiple, elongated and branched buds, beta(1,3)glucan-poor regions of the cell periphery and lysed upon osmotic shock after treatment with the chitin synthase III inhibitor Nikkomycin Z. Ultrastructural examination of cak1P212S mutants revealed a thin, uneven cell wall and marked abnormalities in septum formation. In all of the above aspects, the cak1P212S mutants are similar to previously described cla4 mutants, suggesting that the cell wall defects are common to mutants with hyperpolarized growth. In cak1P212S mutants, chitin accumulates all over the surface of the cells and glucan synthase activity is located preferentially to the tips of elongated buds. We conclude that the cell wall weakness in cak1P212S mutants is caused by hyperpolarized secretion of glucan synthase and lack of reinforcement of the lateral cell walls. Showing that the defect depends at least in part on Cdc28, the cak1P212S hyperpolarized growth phenotype can be suppressed by a Cak1-independent Cdc28-allele. The results underline the importance of a minor cell wall component, the chitin of lateral walls, for the integrity of the cell in a stress situation.