Protein-protein associations in reticulocytes and erythrocytes. (A) Spectrin dimer-tetramer equilibrium in reticulocytes and mature red blood cells. Extracted spectrin was analyzed by nondenaturing gel electrophoresis. Note spectrin from both the reticulocytes and the erythrocytes is tetrameric. In panels B and C, incorporation of a β-spectrin fragment into reticulocyte and erythrocyte membranes. The Triton shells were prepared from reticulocyte or erythrocyte ghosts resealed with increasing concentrations of β-spectrin fragment 1-301. (B) Proteins retained in the Triton shells were analyzed by SDS-PAGE. (C) Quantitative analysis revealed greater incorporation of the peptide into the cytoskeleton of reticulocytes compared with that observed in erythrocytes.