Résumé
Using membrane preparations of the interferon receptor, prepared from cells of the Burkitt line, Daudi, we have examined the binding of three human recombinant α‐interferons. We discovered a binding titration of the interferons IFN‐αA and IFN‐αD in the pH range 6–9. Receptor binding, negligible at pH 6, rises to a maximum close to pH 9. We have shown that binding of IFN‐αA at basic pH is to the same receptors as at neutrality and that IFN‐receptor complexes extracted with digitonin are more stable at basic pH than they are at neutrality. The recombinant interferon, IFN‐αB, shows little change of binding in the pH range 6–9. At its basic optimum the binding of IFN‐αA approaches that of IFN‐αB, while at neutral pH the binding of IFN‐αA is 3–4 times less. This difference at neutral pH is seen on intact cells as well as on membrane preparations. The specific activity of IFN‐αB is close to that of IFN‐αA, both of which are 10–20 times more active than IFN‐αD; and the binding titration is, therefore, independent of the initial binding affinities. Using hybrid IFNs constructed from the DNA sequences of αD and αB, we have isolated the sequence responsible for the binding titration to the segment comprising amino acids 61–92. Examination of these sequences reveals that Lys‐84 is present in all the IFN‐α except IFN‐αB where it is replaced by Glu; and Tyr‐90, present in most of the common IFN‐α including αA and αD, is replaced by Asp in IFN‐αB. Lys and Tyr would normally titrate in the pH range 6–9. We conclude that the binding titration is due to an electrostatic interaction and we propose that the interaction is between IFN‐receptor complexes. The role of the interaction in the binding losses that accompany the antiproliferative effects of IFN is discussed.