Transcriptional responses to secretion stress in the fungi Trichoderma reesei and S. cerevisiae reveal essential differences and common features

Trichoderma reesei is an industrial protein production host known for its exceptional protein secretion capability. This study aims at uncovering the transcriptional responses occurring in T. reesei cells exposed to secretion stress and comparing these responses to similar experiments carried out in S. cerevisiae. Secretion stress is caused by compromised protein folding or transport in the secretory pathway. It induces a number of genes involved in different aspects of secretion through the unfolded protein response (UPR) pathway. In T. reesei it has also been shown that secretion stress down-regulates genes encoding secreted proteins. We constructed cDNA subtraction libraries and made cDNA-AFLP (amplified fragment length polymorphism) experiments from cells under secretion stress. A transformant expressing human tissue plasminogen activator (tPA), treatment with the chemical DTT (dithiothreitol) that prevents correct protein folding and a transformant over-expressing IREI protein (sensor protein of the UPR pathway) were analysed. Around two hundred unique ESTs were retrieved by these methods and the expression pattern of about 50 was confirmed by Northern experiments. A rank sum test for the Northern data was used to define those genes that show upregulation in all the three conditions. Data from DTT and tunicamycin treatment, foreign protein production and IRE1 and HAC1 (UPR transcription factor) deletion experiments in S. cerevisiae were combined from litterature. The transcriptional responses of T. reesei and S. cerevisiae show clear overlap, especially with respect to genes involved in protein translocation, folding and glycosylation in the ER. However, there seems to be major differences in regulation of amino acid biosynthesis and nucleosome genes. The GCN4/CPC1 transcription factor and a limited set of its putative target genes are induced only in T. reesei. This response points to the upregulation of glutathione synthesis to relieve oxidative stress caused by compromised protein folding. Interestingly also a set of nucleosome genes is upregulated in T. reesei without a clear connection to cell cycle.