Phosphorylation of 4.1R and adducin in reticulocytes and erythrocytes. Reticulocyte and erythrocyte ghosts were prepared in the presence of the phosphatase inhibitor, calyculin A. Western blots of SDS-PAGEs were probed with phospho-residue specific antibodies as indicated. PMA-treated erythrocyte ghosts were used as a positive control. Note the markedly increased phosphorylation of 4.1R at residue 331 serine (4.1R s331) in reticulocytes.