MULTIPLE ROLES OF THE CELLULASE CBHI IN ENHANCING PRODUCTION OF FUSION ANTIBODIES BY THE FILAMENTOUS FUNGUS TRICHODERMA-REESEI

The production of Fab antibody fragments in Trichoderma reesei can be increased over 50-fold by fusing the core-linker region of the T. rees ei cellulase CBHI (cellobiohydrolase I) to the heavy Fd chain (Nyysson en et al. 1993). This beneficial role of CBHI in antibody production h as now been studied further by comparisons of T. reesei strains produc ing the light chain only, Fab or CBHI-Fab all of which exhibited ident ical light chain integration. The N-terminal fusion of CBHI to the hea vy Fd chain not only aided secretion, as expected, but also increased the level of mRNA encoding the CBHI-heavy Fd chain, either by stabiliz ing the messenger or by enhancing transcription. The CBHI part appeare d to facilitate secretion at least by aiding the passage through the e ndoplasmic reticulum, since processing of the signal peptide of the an tibody chains seemed to be most efficient in the strain producing CBHI -Fab in contrast to the strains producing light chain or Fab fragment. Interestingly, CBHI core-linker protein, originating from the CBHI-he avy Fd chain, was found in large amounts in the culture medium. The cl eavage resulting in this tailless CBHI occurred inside the cell. This suggests that, by omitting the heterologous tail, the secretion of the resulting CBHI core-linker protein is enhanced to a level comparable with secretion of the extracellular T. reesei proteins.