Coimmunoprecipitation of band 3 and protein 4.2 from metabolically labeled oocytes. Oocytes were injected with either 5 ng of B3 cRNA or 5 ng BM cRNA alone, or 5 ng B3 cRNA or BM cRNA were coinjected with 5 ng of protein 4.2, protein 4.2 Nippon (4.2N), protein 4.2 Tozeur (4.2T), or protein 4.2 Komatsu (4.2K) cRNA. Control oocytes were either injected with 5 ng protein-4.2 cRNA alone or left uninjected. Oocytes were incubated with ³⁵[S]-labeled methionine for 48 hours and then chased with unlabeled amino acids for 4 hours. ³⁵S-methionine–labeled proteins were immunoprecipitated using the B3 C-terminal antibody BRIC155 from homogenates derived from the equivalent of 5 oocytes. In each case ³⁵S-methionine–labeled cell-free–translated (CFT) B3, BM, or protein 4.2 were also separated on duplicate SDS-PAGE gels to provide an indication of size. Panel A shows the results of the coimmunoprecipitation when using B3 run on an 8% SDS-PAGE gel. This shows that both protein 4.2 and protein 4.2 Nippon coimmunoprecipitate with band 3, but markedly less protein 4.2 Tozeur and no protein 4.2 Komatsu was coimmunoprecipitated under the same conditions. Panel B shows the results of coimmunoprecipitation using BM on a 10% SDS-PAGE gel. This shows that the ability to coimmunoprecipitate protein 4.2 with band 3 required the N-terminal cytoplasmic domain of band 3. Similar results to those shown in panel A and panel B have been observed in at least 3 independent experiments and the immunoprecipitations were carried out in duplicate for each experiment.