CONSTRUCTION OF A STARCH-UTILIZING YEAST BY CELL-SURFACE ENGINEERING

We have engineered the cell surface of the yeast Saccharomyces cerevis iae by anchoring active glucoamylase protein on the cell wall, and we have endowed the yeast cells with the ability to utilize starch direct ly as the sole carbon source, The gene encoding Rhizopus oryzae glucoa mylase with its secretion signal peptide was fused with the gene encod ing the C-terminal half (320 amino acid residues from the C terminus) of yeast alpha-agglutinin, a protein involved in mating and covalently anchored to the cell wall. The constructed plasmid containing this fu sion gene was introduced into S. cerevisiae and expressed under the co ntrol of the glyceraldehyde 3-phosphate dehydrogenase promoter from S. cerevisiae, The glucoamylase activity was not detected in the culture medium, but it was detected in the cell pellet fraction, The glucoamy lase protein transferred to the soluble fraction from the cell wall fr action after glucanase treatment but not after sodium dodecyl sulfate treatment, indicating the covalent binding of the fusion protein to th e cell wall, Display of the fused protein was further confirmed by imm unofluorescence microscopy and immunoelectron microscopy. The transfor mant cells could surely grow on starch as the sole carbon source, Thes e results showed that the glucoamylase was anchored on the cell wall a nd displayed as its active form. This is the first example of an appli cation of cell surface engineering to utilize and improve the metaboli c ability of cells.