Expression and characterization of rat UDP-N-acetylglucosamine:  -3-D-mannoside  -1,2-N-acetylglucosaminyltransferase I in Saccharomyces cerevisiae

The yeast Saccharomyces cerevisiae is a useful host for the production of heterologous proteins through the secretory pathway. However, because of the potential antigenicity of mannan-type sugar chains in humans, yeast cannot be used as a host for the production of glycoprotein therapeutics. To overcome this problem, we are trying to breed a yeast which can produce hybrid- or complex-type carbohydrates. UDP- N- acetylglucosamine:alpha-3-d-mannoside beta-1, 2- N- acetylglucosaminyltransferase I (GnT-I) is essential for the conversion of high mannose-type N- glycans to hybrid- and complex-type ones. As yeast lacks this enzyme, we have introduced the rat GnT-I cDNA into yeast cells. The transformed yeast cells expressed GnT-I activity in vitro. The expressed GnT-I was localized in all organella, including the endoplasmic reticulum (ER), Golgi apparatus, and vacuole, suggesting that the mammalian Golgi retention signal of GnT-I did not function in yeast cells. Analysis of the GnT-I gene product with a c-Myc epitope tag at the C-terminus elucidates that the N - terminal region of GnT-I, including the mammalian Golgi retention signal, should be removed in the yeast ER.