On the basis of both functional and structural considerations, members of
the IκB family of proteins can be divided into four groups. The first of these
groups, IκB-α (a 35-37 kDa protein), includes the avian protein pp40 and the
mammalian MAD-3, both of which inhibit binding of p50-p65 NFκB complex
or Rel protein to their cognate binding sites but do not inhibit the binding of
p50 homodimer to κB sites, suggesting that the IκB-α family binds to the
p65 subunit of p50-p65 heterocomplex through ankyrin repeats. The second
member of the IκB family is represented by a 45 kDa protein designated
IκB-β. The third group of IκB proteins is represented by IκB-γ, a 70 kDa protein
identical in sequence with the C-terminal domain of the p110 precursor of
NFκB p50 and expressed predominantly in lymphoid cells.