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In many fungal species, cell growth and morphology, thermo- and osmotolerance are regulated by mitogen-activated protein kinase (MAPk) cascades. Ste11p is a MAP kinase kinase kinase, which plays a central role in various pathways in Saccharomyces cerevisiae. Here we describe ASTE11, an STE11 homologue from Arxula adeninivorans, an imperfect, dimorphic, but nonpathogenic and extremophilic yeast. ASTE11 lacks introns, and codes for a protein of 824 amino acids with a predicted molecular weight of 91.6 kDa. The gene is constitutively expressed at low levels, but is induced by high-salt stress. To facilitate functional analysis of ASTE11, disruption and overexpressing mutants were constructed. The phenotypes of these strains indicate that Aste11p is involved in regulating aspects of cell wall structure and form, but is dispensable for adaptation to hypertonic stress. Despite its structural homology to STE11, ASTE11 cannot complement the mating defect of S. cerevisiae ste11 mutants. These findings emphasize that, although components of MAPk pathways are conserved among yeasts, they often operate in different contexts in different species.

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