Detection of a variant of protein 3, the major transmembrane protein of the human erythrocyte

A variant of the major transmembrane protein of the human erythrocyte has been detected following proteolytic digestion of intact erythrocytes. Pronase digestion of normal erythrocytes gives rise to a 60,000 molecular weight fragment of Protein 3, while digestion of erythrocytes with the variant protein produces two fragments of 60,000 and 63,000 molecular weight when peptides are separated by sodium dodecyl sulfate-acrylamide gel electrophoresis using the discontinuous buffer system of Laemmli (Laemmli, U. K. (1970) Nature 227, 680-685). The two fragments cannot be resolved if electrophoresis is conducted using the continuous phosphate or Tris/acetate buffer systems. This increased molecular weight of the variant fragment does not appear to be due to increased glycosylation, since neither sialic acid residues nor terminal galactose units can be detected. Furthermore, the transmembrane segment of Protein 3 can be detected after proteolytic digestion at both the external and cytoplasmic membrane surfaces. These transmembrane segments of both the normal and the variant peptide have identical molecular weights of 20,000 to 21,000. These results suggest that the increased molecular weight of the variant peptide is due to the incorporation of an additional segment into that region of the molecule which is exposed at the cytoplasmic side of the membrane.