The RNA-binding protein Psc1 functions downstream of the NDR/LATS kinase Cbk1 to modulate CO₂ tolerance in...

Cryptococcus neoformans is an opportunistic fungal pathogen responsible for approximately 20% of deaths in patients with HIV/AIDS. Adaptation to host physiological conditions, including high CO₂, is required for infection. We discovered that an uncharacterized protein with an RNA-binding domain, Psc1, functions as a basidiomycete-specific suppressor of the kinase mutant cbk1Δ and partially rescues its growth defect in high CO₂. Psc1 contains multiple consensus cell wall biosynthesis kinase 1 (Cbk1) phosphorylation sites that are required for maintenance of CO₂ fitness. We hypothesized that, in the absence of Cbk1, Psc1 negatively regulates CO₂ tolerance by binding to and interfering with the function of mRNAs required for CO₂ tolerance. Supporting this model, we found that multiple mRNAs that are required for CO₂ tolerance associate with Psc1 in the absence of Cbk1. Furthermore, the transcripts of ZDS3, a gene required for CO₂ tolerance, predominantly colocalize with Psc1, which forms condensates in the cbk1Δ mutant at high CO₂, correlating with impaired growth. Collectively, our findings support the conclusion that C. neoformans adapts to high CO₂ through a post-transcriptional mechanism where Cbk1 phosphorylates Psc1, preventing its binding to, and subsequent functional inhibition of, mRNAs important for CO₂ tolerance.IMPORTANCEHigh CO₂ is growth-inhibitory, and therefore adaptation to high CO₂ stress is a universal cellular phenomenon required for survival. The environmental fungus, Cryptococcus neoformans, encounters high CO₂ environments in the host, and thus tolerance to high CO₂ is critical for its ability to cause lethal infections. The regulation of Ace2p and morphogenesis (RAM) pathway is required for its growth in high CO₂. We found that CO₂ tolerance is mediated post-transcriptionally by a downstream target of the RAM pathway, the RNA-binding protein Psc1. In the absence of the RAM pathway, Psc1 sequesters mRNA transcripts required for growth in high CO₂, preventing growth. These findings reveal a previously uncharacterized post-transcriptional regulatory mechanism that controls cryptococcal adaptation to high CO₂.