In Candida albicans phosphorylation of Exo84 by Cdk1-Hgc1 is necessary for efficient hyphal extension.

The exocyst, a conserved multiprotein complex, tethers secretory vesicles before fusion with the plasma membrane; thus it is essential for cell surface expansion. In both Saccharomyces cerevisiae and mammalian cells cell surface expansion is halted during mitosis. In S. cerevisiae phosphorylation of the exocyst component Exo84 by Cdk1-Clb2 during mitosis causes the exocyst to disassemble. Here we show that the hyphae of the human fungal pathogen Candida albicans continue extending throughout the whole of mitosis. We show that CaExo84 is phosphorylated by Cdk1, which is necessary for efficient hyphal extension. This action of Cdk1 depends on the hyphal-specific cyclin Hgc1, the homologue of G1 cyclins in budding yeast. Phosphorylation of CaExo84 does not alter its localization, but alters its affinity for phosphatydylserine allowing it to recycle at the plasmamembrane. The different action of Cdk1 on CaExo84 and ScExo84 is consistent with the different locations of the Cdk1 target sites in the two proteins. Thus, this conserved component of polarised growth has evolved so that its phospho-regulation mediates the dramatically different patterns of growth shown by these two organisms.