Cellular responses to secretion stress in Trichoderma reesei

Trichoderma reesei is known for its extremely high capacity of protein secretion. High loads of protein, and especially foreign protein, in the secretory pathway form a challenge to the production organism and expose it to secretion stress. Unfolded protein response (UPR) denotes the induction mechanism of genes encoding ER-resident chaperones and foldases and numerous other genes involved in protein secretion. This induction is triggered when unfolded proteins accumulate into the ER. We have cloned the transcription factor involved in UPR induction, HACI, from T. reesei. Our results indicate that the hac1 gene is activated by a dual mechanism operational at the mRNA level. This mechanism includes a splicing event of an unconventional intron of only 20 nt in length and a truncation of the mRNA at the 5' flanking region. This truncation removes an upstream open reading frame from the mRNA, and we have shown that these uORFs are involved in translational control of the HAC1 protein formation. We have observed that concurrently with the induction of the UPR pathway, the genes encoding secreted proteins are rapidly down-regulated in Trichoderma reesei. This type of regulation can be caused by different secretion inhibitors and by foreign protein expression. The down-regulation is dependent on the promoter of the affected gene, suggesting that it is functional at the transcriptional level. The down-regulation of genes encoding secreted proteins during secretion stress has not been reported before from any other experimental system and thus it could be unique for filamentous fungi.