Peptide-specific antibodies as probes of the orientation of the glucose transporter in the human erythrocyte membrane

Abstract

Antibodies were raised in rabbits against synthetic peptides corresponding to the N-terminal (residues 1-15) and the C-terminal (residues 477-492) regions of the human erythrocyte glucose transporter. The antisera recognized the intact transporter in enzyme-linked immunosorbent assays (ELISA) and Western blots. In addition, the anti-C-terminal peptide antibodies were demonstrated, by competitive ELISA and by immunoadsorption experiments, to bind to the native transporter. Competitive ELISA, using intact erythrocytes, unsealed erythrocyte membranes, or membrane vesicles of known sidedness as competing antigen, showed that these antibodies bound only to the cytoplasmic surface of the membrane, indicating that the C terminus of the protein is exposed to the cytoplasm. On Western blots, the anti-N-terminal peptide antiserum labeled the glycosylated tryptic fragment of the transporter, of apparent Mr = 23,000-42,000, showing that this originates from the N-terminal half of the protein. The anti-C-terminal peptide antiserum labeled higher Mr precursors of the Mr = 18,000 tryptic fragment, although not the fragment itself, indicating that the latter, with its associated cytochalasin B binding site, is derived from the C-terminal half of the protein. Antiserum against the intact transporter recognized the C-terminal peptide on ELISA, and the Mr = 18,000 fragment but not the glycosylated tryptic fragment on Western blots.