The Candida albicans Cdk8-dependent phosphoproteome reveals repression of hyphal growth through a Flo8-dependent pathway

Ssn3, also known as Cdk8, is a member of the four protein Cdk8 submodule within the multi-subunit Mediator complex involved in the co-regulation of transcription. In Candida albicans, the loss of Ssn3 kinase activity affects multiple phenotypes including cellular morphology, metabolism, nutrient acquisition, immune cell interactions, and drug resistance. In these studies, we generated a strain in which Ssn3 was replaced with a functional variant of Ssn3 that can be rapidly and selectively inhibited by the ATP analog 3-MB-PP1. Consistent with ssn3 null mutant and kinase dead phenotypes, inhibition of Ssn3 kinase activity promoted hypha formation. Furthermore, the increased expression of hypha-specific genes was the strongest transcriptional signal upon inhibition of Ssn3 in transcriptomics analyses. Rapid inactivation of Ssn3 was used for phosphoproteomic studies performed to identify Ssn3 kinase substrates associated with filamentation potential. Both previously validated and novel Ssn3 targets were identified. Protein phosphorylation sites that were reduced specifically upon Ssn3 inhibition included two sites in Flo8 which is a transcription factor known to positively regulate C. albicans morphology. Mutation of the two Flo8 phosphosites (threonine 589 and serine 620) was sufficient to increase Flo8-HA levels and Flo8 dependent activity, suggesting that Ssn3 kinase activity negatively regulates Flo8. Previous work has also shown that loss of Ssn3 activity leads to increased alkalinization of medium with amino acids. Here, we show that FLO8 and STP2, a transcription factor involved in amino acid utilization, are required for ssn3{Delta}/{Delta} phenotype, but that loss of the Ssn3 phosphosites identified in Flo8 was not sufficient to phenocopy the ssn3{Delta}/{Delta} mutant. These data highlight the spectrum of processes affected by the modulation of Ssn3 activity and underscore the importance of considering Ssn3 function in the control of transcription factor activities.