STE11 disruption reveals the central role of a MAPK pathway in dimorphism and mating in Yarrowia lipolytica.

Yarrowia lipolytica is a dimorphic fungus whose morphology is controlled by several factors such as pH and different compounds. To determine if the STE11-mitogen-activated protein kinase (MAPK) pathway plays a role in dimorphism of Y. lipolytica, we isolated the gene encoding a Mapkkk. The isolated gene (STE11) has an ORF of 2832 bp without introns, encoding a protein of 944 amino acids, with a theoretical Mr of 100.9 kDa, that exhibits high homology to fungal Mapkkks. Disruption of the STE11 gene was achieved by the pop-in/pop-out procedure. Growth rate and response to osmotic stress or agents affecting wall integrity were unaffected in the deleted mutants, but they lost the capacity to mate and to grow in the mycelial form. Both alterations were reverted by transformation with the wild-type STE11 gene. The Y. lipolytica STE11 gene driven by two different promoters was unable to complement Saccharomyces cerevisiae ste11Delta mutants, although the gene was transcribed. Also, a wild-type MAPKKK gene from Ustilago maydis failed to complement Y. lipolyticaDeltaste11 mutants. Both negative results were attributed to a failure of the transgenic gene products to interact with the corresponding regulatory and scaffold proteins. This hypothesis was supported by the observation that a truncated version of the U. maydis MAPKKK gene reversed mating and dimorphic defects in the mutants. All these results demonstrate that the MAPK pathway is essential for both morphogenesis and mating in Y. lipolytica.