THE ACTIVITY OF A 70 KILODALTON I-KAPPA-B MOLECULE IDENTICAL TO THE CARBOXYL-TERMINUS OF THE P105 NF-KAPPA-B PRECURSOR IS MODULATED BY PROTEIN-KINASE A

The p50 subunit of NF-kappaB is derived from the amino terminus of a 1 05 kilodalton precursor. The p105 carboxyl terminus, which contains an kyrin-like repeats, a feature of IkappaB molecules, regulates the cyto plasmic retention of p105 and inhibits DNA binding by the precursor. H ere, we describe an IkappaB protein identical to the carboxyl-terminal region of p105. Probes spanning the COOH terminus but not the rel hom ology domain of p105 hybridize to a distinct 2.6-kilobase mRNA express ed in a wide range of murine tissues. The nucleotide sequence of compl ementary DNA clones for this transcript, in vitro translation, and imm une precipitation of metabolically labeled cell lysates establish that it encodes a 70 kilodalton protein that corresponds to the COOH-termi nal 607 amino acids of p105. p70 suppresses p65 and p75c-rel mediated trans-activation of reporter genes under the control of NF-kappaB elem ents and in vitro can prevent DNA binding of p50 and p75c-rel homodime rs to NF-kappaB sites. The ability of p70 to stably associate with p49 and p65 in vitro, but not inhibit DNA binding by these proteins, sugg ests that the specific inhibitory properties of this IkappaB may refle ct its relative affinity for different rel targets. p70 phosphorylated by protein kinase A fails to inhibit DNA binding by p50 or the c-rel protein, and sequencing of radiolabeled p70 tryptic phosphopeptides es tablishes that protein kinase A phosphorylates serine residue 576 of p 70. This finding suggests that the inhibitory activity of p70 can be r egulated by signaling via the adenylate cyclase pathway.