The Candida albicans transcription factor Efg1 regulates in vitro filamentation through a cAMP-Protein Kinase A independent mechanism
Candida albicans is one of the most common causes of human fungal disease. One of the most studied C. albicans virulence traits is its ability to adopt both yeast and filamentous morphologies. Accordingly, the regulation of the yeast-to-filament transition has been area of intense study in medical mycology. A longstanding mechanistic paradigm in this field is that the cAMP-protein kinase A pathway phosphorylates a master transcriptional regulator of C. albicans filamentation, Efg1, to drive filamentation. This model has been generalized over the years despite the fact that the initial work showed that it applied to only specific in vitro filamentation conditions. Here, we reinvestigated the protein kinase A-Efg1 paradigm by generating new strains containing alleles of EFG1 with the putative protein kinase A phosphorylation site (T208) mutated to either Ala or Glu which blocks or mimics phosphorylation, respectively. We integrated these alleles into efg1?? mutants so that they are expressed from the endogenous promoter. We assayed the effect of these Efg1 mutations on: 1) in vitro filamentation under a wide range of inducing condition; 2) biofilm formation; 3) in vivo filamentation; 4) virulence in a model of disseminated candidiasis; and 5) gene expression during in vitro filamentation. We found no evidence that blocking or mimicking protein kinase phosphorylation of Efg1 affected its function during filamentation, biofilm formation, of infection. These observations indicate that the protein kinase A-Efg1 model for the regulation of C. albicans filamentation should be reconsidered. Overall design: RNA-seq profiling of wild-type C. albicans and strains containing an EFG1 (T208A) mutation or deletion.