The endoplasmic reticulum (ER) responds to errors in protein folding or processing by induction of the unfolded protein response (UPR). During conditions of ER stress, unconventional splicing of an mRNA encoding the UPR-responsive transcription factor occurs at the ER surface, resulting in activation of the UPR. UPR activation is necessary for adaptation to ER stress and for the pathogenic fungus Cryptococcus neoformans is an absolute requirement for temperature adaptation and virulence. In this study, we have determined that C. neoformans has co-opted a conserved PUF RNA binding protein to regulate the posttranscriptional processing of the HXL1 mRNA encoding the UPR transcription factor. PUF elements were identified in both the 5= and 3= untranslated regions of the HXL1 transcript, and both elements bound Puf4. Deletion of PUF4 resulted in delayed unconventional splicing of HXL1 mRNA and delayed induction of Hxl1 target genes. In addition, the HXL1 transcript was stabilized in the absence of Puf4. The puf4⌬ mutant exhibited temperature sensitivity but was as virulent as the wild type, despite a reduction in fungal burden in the brains of infected mice. Our results reveal a novel regulatory role in which a PUF protein influences the unconventional splicing of the mRNA encoding the UPR-responsive transcription factor. These data suggest a unique role for a PUF protein in controlling UPR kinetics via the posttranscriptional regulation of the mRNA encoding the UPR transcription factor Hxl1.
Cryptococcus neoformans is one of the few fungal species that has evolved to successfully transition from its environmental niche to cause deep infections in humans, which requires adaptation to the mammalian core temperature. The mammalian host temperature exerts endoplasmic reticulum (ER) stress on C. neoformans, leading to a transient induction of the unfolded protein response (UPR) during temperature adaptation (1). The loss of UPR induction through deletion of the stress response mediators IRE1 and HXL1 impairs virulence and temperature adaptation (2). Likewise, prolonged ER stress resulting from the loss of mRNA degradation in a ccr4⌬ mutant also impairs temperature adaptation (1). Together, this information suggests that a balance of UPR induction and resolution is critical for successful temperature adaptation in C. neoformans.The UPR is induced in response to ER stress caused by aberrant protein folding or processing (3-5). A well-conserved feature of UPR activation is the spliceosome-independent removal of an unconventional intron from the mRNA encoding the major UPR transcription factor by the ER-resident transmembrane kinase/ RNase Ire1 (5, 6). In Saccharomyces cerevisiae, this transcription factor is encoded by the HAC1 gene and the unconventional splicing event removes a 252-nucleotide intron from the mRNA, altering the reading frame and allowing production of a functional Hac1p transcription factor. In higher eukaryotes, the orthologous active protein Xbp1 is produced after removal of a 26-nucleotide intron from t...