Heterologous expression of the multi-functional cellulase gene (mfc) from the mollusc Ampullaria crossean, in Volvariella volvacea

Volvariella volvacea, or straw mushroom, is a widely cultivated and important edible fungus. In this paper, we used straw mushroom as a host to express an exogenous mfc gene encoding a multi-functional cellulase (MFC) cloned from the mollusc Ampullaria crossean. The purpose was to increase expression of MFC in the new host, as well as to improve cellulose degradation and increase future yields of V. volvacea. MFC is an enzyme with exo-β-1,4-glucanase, endo-β-1,4-glucanase, and endo-β-xylanase activities. The A. crossean mfc gene was expressed in V. volvacea under the control of an endogenous promoter (gpd-Vvs) using the expression vector pgVvs-mfc, constructed by ligating the gpd-Vvs promoter with the mfc gene. The expression plasmid, pgVvs-afp, containing an afp gene encoding an anti-freeze protein (AFP) cloned from spruce budworm as the selective marker gene, was co-transformed into protoplasts of V. volvacea along with the vector pgVvs-mfc using polyethylene glycol (PEG)-mediated transformation. Putative transformants of V. volvacea were obtained by exposing the mycelia to regeneration medium (200 g l⁻¹ potato extract, 20 g l⁻¹ dextrose, 20 g l⁻¹ agar, and 0.8 M D-mannitol) at 0°C. PCR and Southern blotting were used to identify positive transformants. The results indicated that the mfc gene had been integrated into the genome of V. volvacea. The total cellulose activity, based on filter paper degradation, and carboxymethyl cellulase and xylanase activities in the transformants were 3.92, 4.75, and 58.99 Units ml⁻¹, respectively. Sodium dodecyl sulphate-polyacrylamide gel electrophoresis showed an approx. 46 kDa band, which was equivalent to the expected size of the MFC protein of A. crossean. Thus, the mfc gene appeared to have been expressed heterologously in V. volvacea.