THE PEST-LIKE SEQUENCE OF I-KAPPA-B-ALPHA IS RESPONSIBLE FOR INHIBITION OF DNA-BINDING BUT NOT FOR CYTOPLASMIC RETENTION OF C-REL OR RELA HOMODIMERS

In most cells, proteins belonging to the Rel/NF-kappa B family of tran scription factors are held in inactive form in the cytoplasm by an inh ibitor protein, I kappa B alpha. Stimulation of the cells leads to deg radation of the inhibitor and transit of active DNA-binding Rel/NF-kap pa B dimers to the nucleus. I kappa B alpha is also able to inhibit DN A binding by Rel/NF-kappa B dimers in vitro, suggesting that it may pe rform the same function in cells when the activating signal is no long er present. Structurally, the human I kappa B alpha molecule can be di vided into three sections: a 70-amino-acid N terminus with no known fu nction, a 205-residue midsection composed of six ankyrin-like repeats, and a very acidic 42-amino-acid C terminus that resembles a PEST sequ ence. In this study we examined how the structural elements of the I k appa B alpha protein correlate,vith its functional capabilities both i n vitro and in vivo. Using a battery of I kappa B alpha mutants, we sh ow that (i) a dimer binds a single I kappa B alpha molecule, (ii) the acidic C-terminal region of I kappa B alpha is not required for protei n-protein binding and does not mask the nuclear localization signal of the dimer, (iii) the same C-terminal region is required for inhibitio n of DNA binding, and (iv) this inhibition may be accomplished by dire ct interaction between the PEST-like region and the DNA-binding region of one of the subunits of the dimer.