In yeast, the N-linked oligosaccharide modification in the Golgi apparatus is initiated by a1,6-mannosyltransferase (encoded by theOCH1gene) with the addition of mannose to the Man8GlcNAc2or Man9GlcNAc2 endoplas-mic reticulum intermediates. In order to characterize its enzymatic proper-ties, the soluble form of the recombinant Och1p was expressed in the methylotrophic yeastPichia pastorisas a secreted protein,