Endogenous lectin from cultured soybean cells: Exposure of the SB-1 lectin on the cell wall

Abstract

Digestion of seed soybean agglutinin with V-8 protease yielded seven distinct fragments ( M r 10,000–20,000) that were well-resolved by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Each individual peptide (F 1 through F 7) was isolated; determination of the amino acid sequence at the NH 2-terminal portion of each peptide established its position in the intact polypeptide of soybean agglutinin. The isolated peptides were used as affinity adsorbents to obtain antibodies that bound individual fragments (anti-F 1 through anti-F 7). These antibody preparations were, in turn, used in immunofluorescence staining of intact cultured soybean (SB-1) cells. Only those antibody preparations that bind to the NH 2-terminal portion (residues 1–124) of the intact soybean agglutinin showed significant cell surface labeling. In contrast, the antibody preparations that bound to residues 125–253 failed to bind to intact SB-1 cells. These results suggest that the SB-1 lectin has the NH 2-terminal portion of the polypeptide chain exposed and accessible at the cell surface, while the COOH-terminal portion of the same molecule may be masked, either through protein folding or through embedding in the cell wall. Limited digestion of the cell wall polysaccharides by cellulase or pectinase released the majority of the cell surface lectin.