Immunogold-localization and synthesis of an oil-body membrane protein in developing soybean seeds

The synthesis of a major oil-body membrane brotein was studied in maturing soybean (Glycine max (L.) Merr.) cotyledons. The membrane contained four abundant proteins with apparent molecular mass (M_r) of 34000, 24000, 18000 and 17000. The M_r=24000 protein (mP 24) was selected for more detailed analysis. The protein was purified to apparent homogeneity by preparative sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and isolated from the gel by electroelution or chemical hydrolysis of gel crosslinks. It was then used to elicit rabbit antibodies which were judged to be specific when assayed by SDS-PAGE-immunoblot procedures. The mP 24 was localized in immature soybean cotyledon cells by indirect immunogold procedures on thin sections of Lowicryl- and LR-White-embedded tissue. Indirect labeling with the primary antiserum followed by colloidal gold-protein A showed specific labeling of the oil-body membrane and an absence of label on the other subcellular organelles including the endoplasmic reticulum (ER). Parallel tissue samples were studied by conventional transmission electron microscopy. Although segments of the ER were observed to be closely juxtaposed to the oil bodies, continuity between the two organelles was not observed. The synthesis of mP 24 was studied by in-vitro translation and in-vivo labeling with [³H]leucine followed by indirect immunoaffinity isolation of the labeled products. The SDS-PAGE fluorography results indicated that the primary translation product and the in-vivo synthesized protein have the same M_r, and this is also the same M_r as the protein in the mature membrane.