The basic 7S globulin (Bg) is a cysteine-rich glycoprotein in soybean seeds. Mature Bg is composed of subunits of high and low molecular mass that are linked by disulfide bonds. Bg and Bg-like proteins from seeds of other legumes and from carrot cells in suspension culture are capable of binding to insulin and to insulin-like growth factors. The ultrastructural localization of Bg in immature soybean cotyledons was investigated immunocytochemically using polyclonal antibodies and immunogold labeling. Bg was localized mainly in the middle lamella of the cell wall on the side facing the intercellular space. A few gold particles were also detected near the plasma membrane, in Golgi vesicles that were fusing with the plasma membrane and in the rough endoplasmic reticulum (rER). These observations suggest that Bg is a novel cell-wall protein in the middle lamella of soybean seeds and that is synthesized in the rER and secreted into the cell wall via Golgi vesicles.
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