Abstract
HAC1 encodes a transcription factor that mediates the unfolded protein response (UPR) in Saccharomyces cerevisiae. We characterized hac1Delta mutants in the sporulation-proficient SK1 genetic background and found a novel function for HAC1 in haploid tolerance. hac1Delta spore clones contain a diploid DNA content as determined by fluorescence-activated cell sorting and genetic analyses. Autodiploidization of hac1 spore clones occurred after germination; hac1 spores were born haploid, but efficiently generated diploid progeny during the subsequent mitotic division. Once the hac1 mutant acquired a diploid DNA content, no further ploidy increase was observed. Interestingly, the increase in genome content following meiosis was not a general property associated with hac1 spore clones; instead, it was restricted to an inability to tolerate the haploid state. Genetic analyses involving the UPR target gene KAR2 and the UPR regulator IRE1 revealed that autodiploidization associated with hac1 mutants is a consequence of its role in the UPR pathway. Inhibition of the UPR pathway induces autodiploidization, and constitutive activation of UPR target genes suppresses this response.
Highlights
The endoplasmic reticulum (ER)1 is a membranous network structure where secretory and trans-membrane proteins are processed by disulfide (S-S) bond formation, glycosylation, folding, and oligomerization
No ploidy increase was observed, indicating that genome duplication was not a general property associated with hac1⌬100::URA3 spore clones; instead, it was restricted to an increase from haploid to diploid, suggesting that hac1⌬ mutants were unable to tolerate the haploid state
Fluorescence-activated Cell Sorting (FACS) analysis verified that hac1⌬100::URA3 spore clones contain a diploid
Summary
Activated Ire1p triggers synthesis of the basic leucine zipper transcription factor Hac1p (homology to ATF/CREB) by cleaving a 252-nucleotide intron from unspliced HAC1u mRNA (u ϭ uninduced) [11, 12]. Splicing of the intron replaces this 10-amino acid tail with a novel 18-amino acid tail encoded by the second exon, generating the 238-amino acid protein, Hac1pu This change increases the transcriptional activation potential of Hac1pi [19]. In addition to its role in the UPR, the Hac1p transcription factor regulates cell differentiation in response to nutrient availability and functions as a positive regulator of the inositol biosynthetic pathway [24, 25]. Inhibition of the UPR pathway results in autodiploidization and overexpression of Kar2p/BiP suppresses this phenotype, suggesting that functional UPR is required for ploidy maintenance
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