Suspension-cultured cells of rice secrete a-amylase into the culture medium. It has been shown that the mature form of the a-amylase contains xylose-bearing N-linked oligosaccharide:GlcNAc(P1-2)Man(al-6) , GlcNAc(P1-2)Man(al-3)Man(pl-4)GlcNAc(~1-4) GlcNAc ' IXYW-2) and GlcNAc(P1-2) Man(a1-6)We demonstrate that suspension-cultured cells of rice secrete a-amylase containing oligomannose-type oligosaccharides in the presence of 1-deoxymannojirimycin or tris(hydroxymethy1)aminomethane. On the other hand, a-amylase purified from germinated rice seedlings contains several kinds of oligomannose-type and Nacetyllactosamine-type oligosaccharides. The processing pathway of oligosaccharide moieties in rice cells is discussed on the basis of a comparison of these oligosaccharides structures.Numerous secretory proteins are known to carry Nacetyllactosamine-type oligosaccharides [l]. The biosynthesis of glycoprotein sugar chains of animal origin have been wellcharacterized. This pathway starts from the transfer of Glc3Man9GlcNAc2 to the nascent polypeptide in the endoplasmic reticulum (ER). The N-acetyllactosamine-type oligosaccharides are subsequently produced by many enzymatic steps during intracellular transport through the ER and Golgi compartments [2].The structure of oligosaccharides in plant glycoproteins has been elucidated during the last decade. The presence of oligomannose-type oligosaccharides in soybean agglutinin having the same structure as that found in animal glycoproteins was reported in 1978 [3]. In 1979, Ishihara et al. [4] Correspondence to N. Takahashi, Nagoya City University, College of Nursing, Mizuho, Nagoya, Japan 467Abbreviations. ER, endoplasmic reticulum; Fuc, fucose; endo H, endo-P-N-acetylglucosaminidase H; Xyl, xylose.Enzymes. a-Amylase (EC 3.2.1 .I); endo-P-N-acetylglucosaminidase H (EC 3.2.1.96); pepsin (EC 3.4.23.1); glycopeptidase A (EC 3.5.1.52); P-N-acetylhexosaminidase (EC 3.2.1.52); a-mannosidase (EC 3.2.1.24); P-xylosidase (EC 3.2.1.37).Note. This is paper no. 23 in the series Enzymatic mechanism of starch breakdown in germinating rice seeds; paper no. 22 is [41].reported the complete structure of an oligosaccharide from pineapple stem, bromelain, which contains xylose (Xyl) and fucose (Fuc) residues. In recent years, many plant glycoproteins have been found to contain Xyl(P1-2) linked to the P-mannose residue and Fuc(a1-3) attached to the reducing GlcNAc residue. This type of structure has not been found in the mammalian glycoprotein molecules [S -121. Although Johnson and Chrispeels [13] and Kimurd et al. [7] discussed possible mechanism(s) of oligosaccharide processing in plant cells, little is known about the enzymatic reactions responsible for the processing of the plant-specific oligosaccharides and their relationship to intracellular secretory transport.We have previously reported biosynthesis, intracellular transport and extracellular secretion of a-amylase in freshly excised scutellar segments of rice seedlings. Our data indicated that there are two types of a-amylase mole...