Effect of Membrane Splitting on Transmembrane Polypeptides

We investigated the effect of membrane splitting on the primary structure of human erythrocyte membrane polypeptides. Monolayers of intact, chemically unmodified cells were freeze-fractured and examined by one-dimensional SDS PAGE. Silver-stained gels revealed all major polypeptides that stain with Coomassie Blue as well as all bands that stain with periodic acid Schiff's reagent. Both nonglycosylated and glycosylated membrane polypeptides could be detected at concentrations of only a few nanograms per band. Membrane splitting had no effect on the position or number of bands. Monolayers of intact erythrocytes that had been enzymatically radioiodinated with lactoperoxidase were examined by electrophoresis, fluorography, and liquid scintillation counting. Radioactivity was quantified before and after monolayer formation and splitting, and at several stages of gel staining, drying, and fluorography. Although overexposed fluorographs revealed several minor radioiodinated bands in addition to band 3 and the glycophorins, no new bands were detected in split membrane samples derived from intact cells. These observations support the conclusion that neither the band 3 anion channel nor the glycophorin sialoglycoproteins are fragmented during freeze-fracturing. Although both band 3 and glycophorin partition to the cytoplasmic side of the membrane, preliminary quantitative observations suggest an enrichment of glycophorin in the split extracellular "half" membrane. We conclude that the process of membrane splitting by planar monolayer freeze-fracture does not cleave the covalent polypeptide backbone of any erythrocyte membrane protein, peripheral or integral.