Behavior of fragmented band 3 from chymotrypsin-treated bovine erythrocyte membrane in nonionic detergent solution

Bovine band 3 which had been cleaved into two membrane intercalated fragments by extracellular chymotryptic digestion was partially purified and the behavior of fragmented band 3 was compared with that of intact band 3 in nonionic detergent, C12E9, solution. The two fragments were always co-eluted with nearly a 1:1 molar ratio from ion-exchange and gel filtration columns, confirming the observation of Reithmeier ((1979) J. Biol. Chem. 254, 3054-3060) that the fragments are bound together. However, as shown by the molecular size of fragmented band 3 and its reactivity with dimethylmaleic anhydride and/or a cross-linking reagent, the fragmentation caused some change in gross conformation of the protein. Dissociation of the two fragments, though not complete, was found to occur on treatment of fragmented band 3 with dimethylmaleic anhydride, which is a dissociating reagent for band 3 oligomers into the monomers. The disruption of self-associated band 3 was accompanied with a disruption of helical structure. The results suggest that a mutual interaction between the chymotryptic fragments is of importance to form the folded structure maintaining a band 3 dimer.