Biosynthesis of rice seed alpha-amylase: two pathways of amylase secretion by the scutellum

Abstract

The alpha-amylase molecule secreted from the scutellar tissues of rice seedlings bears asparagine-linked oligosaccharides which include both (modified) complex-type and high-mannose-type structures. On the basis of their sensitivity to endo-beta-N-acetylglucosaminidase (Endo-beta-H), they are designated as R and S types. When labeled with [3H]fucose a typical R-type alpha-amylase is labeled. By contrast, [3H]mannose-labeled alpha-amylase can be partly digested by Endo-beta-H; hence, it contains both R and S molecules. The role of the Golgi complex in the post-translational oligosaccharide maturation of alpha-amylase was explored by use of the carboxylic ionophore, monensin (10(-7)M), a known perturbant of the structure and function of the Golgi complex. The monensin sensitivity of alpha-amylase transport and acquisition of terminal sugars as well as the morphologic consequences of monensin treatment point to a similarity between the Golgi complex of plant and animal cells. In order to elucidate the relationship between the secretion of two different forms of alpha-amylase and the partial inhibitory effect exerted by monensin, the possible role of Ca2+ in the secretory pathway was examined. The secretion of the R form was stimulated by Ca2+, whereas that of the S form was not affected by the external concentration of Ca2+. In pulse-chase experiments, we found that R-type alpha-amylase accumulates intracellularly under Ca2+-free conditions. These results indicate that there is both Ca2+-dependent and Ca2+-independent secretion of alpha-amylase in the rice scutellar epithelium cells.