Localization of expression and secretion of a heterologous barley cysteine endopeptidase (EPB) and the homologous main cellobiohydrolase I (CBHI) in a Trichoderma reesei transformant expressing both proteins were studied. The transformant was grown on solid medium with Avicel cellulose and lactose to induce the cbh1 promoter for the synthesis of the native CBHI and the recombinant barley protein linked to a cbh1 expression cassette. Differences in localization of expression between the two proteins were clearly indicated by in situ hybridization, indirect immunofluorescence, and immunoelectron microscopy. In young hyphae, native-size recombinant epb mRNA was localized to apical compartments. In older cultures, it was also seen in subapical compartments but not in hyphae from the colony center. The recombinant EPB had a higher molecular weight than the native barley protein, probably due to glycosylation and differential processing in the fungal host. As was found with its transcripts, recombinant EPB was localized in apical and subapical compartments of hyphae. The cbh1 mRNA and CBHI were both localized to all hyphae of a colony, which suggests that the endogenous CBHI was also secreted from these. In immunoelectron microscopy, the endoplasmic reticulum and spherical vesicles assumed to contribute to secretion were labeled by both CBHI and EPB antibodies while only CBHI was localized in elongated vesicles close to the plasma membrane and in hyphal walls. The results indicate that in addition to young apical cells, more mature hyphae in a colony may secrete proteins.
