The ability of filamentous fungi to secrete high levels of glycosylated proteins has led to an interest in exploiting these organisms as hosts for the production of recombinant chemotherapeutic proteins. However, it is apparent that secreted yields of heterologous proteins are significantly lower than yields of native proteins (Peberdy, Trends in Biotechnology 12:50-57, 1994). Work is being carried out to express heterologous proteins in Aspergillus niger by identifying possible bottlenecks in the secretion process. An aspect of this involves the study of quality control in the glycosylation pathway of secreted proteins. Calnexin and calreticulin are lectins that function as molecular chaperones in the endoplasmic reticulum. These proteins recognise the terminal glucose residues on glycoproteins and prevent their secretion from the cell if the molecule is incorrectly processed. Together with UDP:glucose glycoprotein glucosyltransferase these chaperones form part of a novel mechanism for promoting folding, oligomeric assembly and quality control in the ER (Helenius et al., Trends in Cell Biology 7: 193-200, 1997). The calnexin gene has been identified in A. niger, and a full genomic clone sequenced which shows approximately 60% identity with other calnexin genes. The promoter region contains unfolded protein response elements that are seen in other chaperones, however data obtained does not support these as being functional. It has not been possible to isolate calreticulin from A. niger, and to date this protein has only been found in higher eucaryotes. The enzyme UDP:glucose glycoprotein glucosyltransferase has a key function in maintaining glycan chains so unfolded proteins are recognisable by chaperones. This gene for this protein has been isolated from A. niger and a genomic clone is being sequenced.
Fungal Genet. Newsl. 46 (Supl):
Full conference title:
Fungal Genetics Conference 20th
- Fungal Genetics Conference 20th (1999)