COUPLING OF A SIGNAL RESPONSE DOMAIN IN I-KAPPA-B-ALPHA TO MULTIPLE PATHWAYS FOR NF-KAPPA-B ACTIVATION

The eukaryotic transcription factor NF-kappa B plays a central role in the induced expression of human immunodeficiency virus type 1 and in many aspects of the genetic program mediating normal T-cell activation and growth. The nuclear activity of NF-kappa B is tightly regulated f rom the cytoplasmic compartment by an inhibitory subunit called I kapp a B alpha. This cytoplasmic inhibitor is rapidly phosphorylated and de graded in response to a diverse set of NF-kappa B-inducing agents, inc luding T-cell mitogens, proinflammatory cytokines, and viral transacti vators such as the Tax protein of human T-cell leukemia virus type 1. To explore these I kappa B alpha-dependent mechanisms for NF-kappa B i nduction, we identified novel mutants of I kappa B alpha that uncouple its inhibitory and signal-transducing functions in human T lymphocyte s. Specifically, removal of the N-terminal 36 amino acids of I kappa B alpha failed to disrupt its ability to form latent complexes with NF- kappa B in the cytoplasm. However, this deletion mutation prevented th e induced phosphorylation, degradative loss, and functional release of I kappa B alpha from NF-kappa B in Tax-expressing cells. Alanine subs titutions introduced at two serine residues positioned within this N-t erminal regulatory region of I kappa B alpha also yielded constitutive repressors that escaped from Tax-induced turnover and that potently i nhibited immune activation pathways for NF-kappa B induction, includin g those initiated from antigen and cytokine receptors. In contrast, in troduction of a phosphoserine mimetic at these sites rectified this fu nctional defect, a finding consistent with a causal linkage between th e phosphorylation status and proteolytic stability of this cytoplasmic inhibitor. Together, these in vivo studies define a critical signal r esponse domain in I kappa B alpha that coordinately controls the biolo gic activities of I kappa B alpha and NF-kappa B in response to viral and immune stimuli.