Heterologous gene expression by filamentous fungi: secretion of human interleukin-6 by Aspergillus nidulans.

Expression vectors for human interleukin-6 (hIL6) contain an expression cassette consisting of the Aspergillus niger glaA promoter and the Aspergillus nidulans argB terminator. The secretion signals were either those of glaA or that of the authentic hIL6 peptide. The constructs under study were introduced into A. nidulans and A. niger by means of cotransformation. No IL6 activity could be detected in the medium of a cotransformed A. niger strain, although transcripts corresponding with the IL6 cDNA were present. Evidence is presented that this apparent lack of IL6 expression is due to extracellular proteolytic activity. In the media of a cotransformed A. nidulans strain grown on starch, IL6 activity was detected by means of a bioassay. Up to 25 ng/ml of biologically active hIL6 could be secreted by A. nidulans transformed with the plasmid containing the mature hIL6-encoding gene fused to the glaA signal peptide nucleotide sequences. hIL6 of the expected 23-kDa size was also observed by Western-blot analysis of the medium. There was no evidence for glycosylation of the protein.