Heterologous expression of T. reesei secretory protein in S. cerevisiae PMT mutants

The saprophytic fungus Trichoderma reesei secretes a wide range of enzymes which are of considerable biotechnological importance. It was postulated that O-mannosylation is necessary for protein secretion in T.reesei. Due to the complicated genetics of the fungus it is very difficult to study this process in T.reesei. Thus to follow an effect of O-mannosylation on the secretion of T.reesei protein CBHII (cellobiohydrolase II) containing a heavily O-mannosylated linker between the catalytic and cellulose binding domains, we have decided to use S.cerevisiae. In S.cerevisiae seven genes, PMT 1-7 ,coding for protein-O-mannosyltransferases, the first enzyme starting synthesis of O-glycosidic linkage sugars, were identified. CBHII was expressed in yeast pmt mutants and in a parental strain. Enzyme activity was detected on the plates containing medium with beta-glucan. Hydrolysis of the substrate was observed only in pmt4 mutant and in parental strain. pmtl and pmt2 mutants bearing cbh2 gene were not exhibiting CBHII activity on the plate. CBHII protein secreted by the pmt4 mutant and the parental strain was glycosylated to the same extend what means that CBHII is not glycosylated by Pmt4p. Our results indicate that most likely Pmt1,2 protein complex is needed for CBHII O-mannosylation. If the O-mannosylation is necessary for protein secretion the protein should be accumulated in the cells of pmtl and pmt2 mutants. However, immunodetection of CBHII in the cells free extracts of the pmtl and pmt2 strains indicates absence of the protein in the cells. In pmt2 mutant this effect was corrected by Trichoderma pmt gene homologous to the yeast PMT2. The latter does not confirm however direct dependence between secretion on O-mannosylation. Thus the proteolytic stability of the non glycosylated CBHII protein is also considered.