A SOYBEAN BINDING-PROTEIN (BIP) HOMOLOG IS TEMPORALLY REGULATED IN SOYBEAN SEEDS AND ASSOCIATES DETECTABLY WITH NORMAL STORAGE PROTEINS IN-VITRO

The endoplasmic reticulum (ER) luminal binding protein (BiP) is though t to be a key mediator of folding and assembly of de novo synthesized secretory proteins. We have used a maize (Zea mays L.) BiP antibody to identify its homolog in soybeans (Glycine max (L.) Merril). The accum ulation of BiP in developing soybean seeds seems to be coordinated wit h the onset of active storage protein synthesis. We used a co-immunopr ecipitation assay to detect soybean BiP: beta-conglycinin interactions . Either a maize BiP antibody or a beta-conglycinin antibody co-immuno precipitated the reciprocal protein from whole seed protein extract en zymatically depleted of adenosine 5'-triphosphate (ATP), while an unre lated antibody failed to immunoprecipitate either one. The association of BiP:beta-conglycinin complexes was completely reversed by addition of ATP, a diagnostic feature of molecular chaperone-mediated interact ion. However, only a very small fraction of beta-conglycinin was found to be associated with BiP in whole cell protein extracts from immatur e seeds. These results are consistent with a transient association bet ween BiP and beta-conglycinin subunits, and suggests its involvement i n the biosynthetic transport pathway of storage proteins to protein bo dies.