Ultrastructural Localization of the Basic 7S Globulin in Soybean (Glycine max) Cotyledons

The basic 7S globulin (Bg) is a cysteine-rich glycoprotein insoybean seeds. Mature Bg is composed of subunits of high andlow molecular mass that are linked by disulfide bonds. Bg andBg-like proteins from seeds of other legumes and from carrotcells in suspension culture are capable of binding to insulinand to insulin-like growth factors. The ultrastructural localizationof Bg in immature soybean cotyledons was investigated immunocytochemicallyusing polyclonal antibodies and immunogold labeling. Bg waslocalized mainly in the middle lamella of the cell wall on theside facing the intercellular space. A few gold particles werealso detected near the plasma membrane, in Golgi vesicles thatwere fusing with the plasma membrane and in the rough endoplasmicreticulum (rER). These observations suggest that Bg is a novelcell-wall protein in the middle lamella of soybean seeds andthat is synthesized in the rER and secreted into the cell wallvia Golgi vesicles. (Received January 6, 1994; Accepted August 12, 1994)