Abstract
The non-conventional yeast Yarrowia lipolytica is a suitable model for the study of yeast dimorphism. In order to identify genes that may be involved in the regulation of this process, random mutagenesis was performed. This led to the isolation of monomorphic mutants that had lost the ability to grow in a hyphal form both in liquid and on solid medium. Filamentation was restored to one of the mutants by transformation with a fragment of Y. lipolytica genomic DNA containing a single 2766-bp ORF. The predicted protein has a molecular weight of 99.6 kDa and is highly homologous to the protein kinases Cla4 of Candida albicans and Saccharomyces cerevisiae, which are members of the p21-activated kinase (PAK) family. Analysis of the putative protein sequence identified conserved C-terminal catalytic, and internal Cdc42p-binding regions, as well as a pleckstrin homology domain typical of PAK kinases. The results indicate that CLA4 is a single-copy gene located on the chromosome V of Y. lipolytica. Deletion of CLA4 is not lethal, but completely eliminates the ability to form filaments and to invade agar. A strain lacking a functional CLA4 gene exhibits an aberrant distribution of chitin in the cell wall, indicating a possible role for the Cla4 protein kinase in the maintenance of cell polarity in Y. lipolytica.