Inhibits NF-kappa-B by complexing with and trapping it in the cytoplasm. Inhibits DNA-binding of NF-kappa-B p50-p65 and p50-c-Rel complexes (By similarity). Interacts with RELA, REL, NFKB1 nuclear factor NF-kappa-B p50 subunit and NFKB2 nuclear factor NF-kappa-B p52 subunit. Serine phosphorylated; followed by proteasome-dependent degradation. Belongs to the NF-kappa-B inhibitor family.