Expression of the seven major protein 4.1R isoforms in transfected COS-7 cells. The major 4.1R isoforms identified by RT-PCR of erythroblast total RNA were expressed as HA-epitope–tagged proteins in transfected COS-7 cells. Proteins were immunoprecipitated from precleared cell lysates using a polyclonal anti–HA-epitope tag antibody and analyzed by SDS-PAGE on a 6% (A) or 7% (B) acrylamide gel. (A) Cells transfected with empty expression vector (lane 1); HA-tagged 4.1R¹³⁵ (lane 2); HA-tagged 4.1R¹³⁵▵E16 (lane 3); HA-tagged 4.1R¹³⁵+E14 (lane 4); and HA-tagged 4.1R¹³⁵▵E4 (lane 5). (B) Cells transfected with empty expression vector (lane 1); HA-tagged 4.1R⁸⁰ (lane 2); HA-tagged 4.1R⁸⁰▵E16 (lane 3); HA-tagged 4.1R⁸⁰▵E5 (lane 4). HA-tagged 4.1R isoforms (←). IgG heavy chains migrating ∼55 kD (+). Fainter bands migrating between the HA-epitope–tagged proteins and IgG heavy chains are likely to represent proteolytic fragments of the epitope tagged proteins (see Results). Note that 4.1R⁸⁰▵E5 is expressed at a much lower level than the other 4.1R isoforms.