Plasticity of septin filament assembly during hyphal development in Candida albicans

Abstract

Candida albicans is a major fungal pathogen in immunologically compromised patients. A key virulence trait is its ability to switch between the yeast and hyphal forms. Whereas yeast cells are required for dissemination, the filamentous forms are important in tissue penetration and invasion. A common feature for filamentous growth among dimorphic fungi is the ability to inhibit cell separation after cytokinesis, although it is poorly understood how this process is developmentally regulated. Previously, we have shown that the inhibition of cell separation in hyphae requires a modification of the dynamic properties of septins, a conserved family of GTPases that normally form a ring at the site of cytokinesis. Septins are GTP‐binding proteins that assemble into hetero‐oligomers and filaments and are important elements in morphogenesis in almost all eukaryotic organisms with the exception of plants. In C. albicans yeasts, septins form a ring at the bud emergence site that persists until cytokinesis, when it splits into two rings and dissasembles. The basic polymerization unit is a hetero‐tetramer, and two of them join into a nonpolar octamer (Cdc11‐Cdc12‐Cdc3‐Cdc10‐Cdc10‐Cdc3‐Cdc12‐Cdc11). Sep7, a fifth septin, can substitute Cdc11 at the terminal position. In hyphae septin rings are assembled inside the germ tube, split into two rings before cytokinesis and they are maintained after cytokinesis, resulting in multiple rings along the length of the hypha. Inhibition of hyphal cell separation intrinsic to hyphal development is dependent on the Sep7 septin and on its phosphorylation by the hyphaspecific cyclin‐CDK complex Hgc1‐Cdc28. sep7Δ mutants form normal hyphae, but the compartments separate after cytokinesis. In this work, we have analyzed the contribution of Sep7 and Cdc11 to the inhibition of hyphal cell separation. The results indicate that the proportion of Sep7/Cdc11 in the filaments is essential to maintain the hyphal compartments attached, since a sep7+/‐ heterozygous mutant has an intermediate phenotype between the WT and the sep7‐/‐ mutant, and this defect is corrected when the Cdc11 dosage is reduced in the sep7+/‐ cdc11+/‐ heterozygous. We have also analyzed the different domains of Sep7 required for normal hyphal development and the putative phosphorylation sites that might contribute to this regulation.