p105·IκBγ and Prototypical IκBs Use a Similar Mechanism to Bind but a Different Mechanism to Regulate the Subcellular Localization of NF-κB*

p105, also known as NF-κB1, is an atypical IκB molecule with a multi-domain organization distinct from other prototypical IκBs, like IκBα and IκBβ. To understand the mechanism by which p105 binds and inhibits NF-κB, we have used both p105 and its C-terminal inhibitory segment known as IκBγ for our study. We show here that one IκBγ molecule binds to NF-κB dimers wherein at least one NF-κB subunit is p50. We suggest that the obligatory p50 subunit in IκBγ·NF-κB complexes is equivalent to the N-terminal p50 segment in all p105·NF-κB complexes. The nuclear localization signal (NLS) of the obligatory p50 subunit is masked by IκBγ, whereas the NLS of the nonobligatory NF-κB subunit is exposed. Thus, the global binding mode of all IκB·NF-κB complexes seems to be similar where one obligatory (or specific) NF-κB subunit makes intimate contact with IκB and the nonobligatory (or nonspecific) subunit is bound primarily through its ability to dimerize. In the case of IκBα and IκBβ, the specific NF-κB subunit in the complex is p65. In contrast to IκBα·NF-κB complexes, where the exposed NLS of the nonspecific subunit imports the complex to the nucleus, p105·NF-κB and IκBγ·NF-κB complexes are cytoplasmic. We show that the death domain of p105 (also of IκBγ) is essential for the cytoplasmic sequestration of NF-κB by p105 and IκBγ. However, the death domain does not mask the exposed NLS of the complex. We also demonstrate that the death domain alone is not sufficient for cytoplasmic retention and instead functions only in conjunction with other parts in the three-dimensional scaffold formed by the association of the ankyrin repeat domain (ARD) and NF-κB dimer. We speculate that additional cytoplasmic protein(s) may sequester the entire p105·NF-κB complex by binding through the death domain and other segments, including the exposed NLS.