THE REL-ASSOCIATED PP40 PROTEIN PREVENTS DNA-BINDING OF REL AND NF-KAPPA-B - RELATIONSHIP WITH I-KAPPA-B-BETA AND REGULATION BY PHOSPHORYLATION

The product of proto-oncogene Rel associates with a number of cellular proteins. We have studied the effect of one of them, a phosphoprotein of 40 kD (pp40), on the DNA-binding activity of the Rel protein. We demonstrate that purified pp40 not only inhibits the binding of Rel, but also NF-kappa-B (p50-p65) heterocomplex to DNA. Additionally, I-kappa-B-beta, but not I-kappa-B-alpha, also prevented the binding of Rel to the kappa-B site. I-kappa-B-beta and pp40 are related proteins because (1) they share a number of common tryptic peptides, (2) their inhibitory effect on DNA binding can be abolished by preincubation with pp40-specific antiserum, and (3) labeled I-kappa-B-beta can be immunoprecipitated with pp40 antibodies. pp40 is part of the Rel complex present in the cytoplasm and nuclear extracts of WEHI-231 cells. The activity of pp40 to inhibit the DNA binding of Rel and NF-kappa-B is modulated by phosphorylation.