Interplay between the NDR kinase Cbk1 and the phosphatase Cdc14 during the developmental programme of Candida albicans

Abstract

Fungi are able to colonize highly divergent niches, ranging from high osmotic enviroments to plant and animal hosts. Their ability to adapt to these hostile enviroments depends on their capability to sense a variety of external cues, transduce the signals to specific cytoplasmic targets and activate the appropiate response. Although S. cerevisiae and C. albicans share some common features, they also exhibit many significant differences since they diverged from a common ancestor more than 500 million years ago. In nature, as a result of this divergence, S. cerevisiae is found in sugar-rich habitats such the surface of fruits, wheras C. albicans is normally found as a commensal in the gastrointestinal tract of humans. Therefore, these distinct life styles might have been a driving force in rewiring signalling networks in both yeasts. C. albicans has several attributes that allow it to adapt rapidly to changing environmental signals that contribute to host colonization. One of the best studied is the ability to switch between different morphologies, such yeast, pseudophyphae and hyphae. The yeast-to-hypha transition, triggered by a wide range of environmental cues, is regulated by multiple signalling pathways that control the transcription of a set of hypha-specific genes (HSCs), many of which encode known virulence factors. Members of the evolutionarily conserved Nuclear Dbf2-Related (NDR) kinase family are essential components of signalling pathways involved in cell morphogenesis and proliferation control from yeast to humans. In C. albicans, the NDR kinase Cbk1 and the Cdc14 phosphatase are essential for hyphal growth. Here we describe that Cbk1 and Cdc14 interact in vivo through the Cbk1 regulatory subunit Mob2. Based on gel mobility assays, we show that Cbk1 is dephosphorylated by Cdc14 upon hyphal induction and subsequently phosphorylated during the maintenance of hyphal growth by the hypha-specific CDK complex Cdc28/Hgc1. By using different phosphodeficient mutants we propose a working model in which the NDR complex Cbk1/Mob2 is activated by the Cdc14 phosphatase to reset the phosphorylation state of the kinase to a satate required for maintaining polarised growth at the tip of hyphal cells.