Mechanisms controlling signal-mediated NF-kappaB nuclear expression and export

The transcription factor NF- k B is normally sequestered in the cytoplasm as an inactive complex by the inhibitory I k B proteins. In response to a large array of extracellular stimuli, signals are transduced intracellularly which trigger the signal-mediated phosphorylation, ubiquitination, and degradation of the I k B proteins with the concomitant nuclear translocation of NF- k B to modulate gene expression via enhancers bearing k B elements. Upon the completion of an NF- k B activation response, NF- k B is shuttled back to the cytosol allowing the system to reset for a subsequent response. In this thesis, studies are provided which elucidate the mechanisms governing the signal-mediated nuclear expression of NF- k B and its subsequent export to the cytosol. Further studies provide insight into the strategies utilized by the retroviral trans-activator proteins Tat and Tax to subvert NF- k B activation to modulate changes in viral or cellular gene expression.; NF- k B is a critical regulator of T cell activation since it controls the expression of both IL-2 and its high affinity receptor. Cell-surface mediated signals activate NF- k B during a physiological T cell activation event whereby both the T cell receptor and costimulatory molecules are engaged. It is demonstrated that the CD28 costimulatory molecule contributes to TCR induced NF- k B activation by enhancing degradation of I k B a and inducing de novo degradation of I k B b which is responsible for a persistent NF- k B response. The CD28 pathway synergizes with the TCR pathway at the level of the I k B kinases as stimulation of T cells through the TCR and CD28 greatly augments the kinase activity of IKK a and IKK b , two recently identified I k B kinases which target I k B a and I k B b for degradation by signal-mediated phosphorylation. Subsequent studies into the termination of an NF- k B response revealed that RelA contains a leucine rich sequence within its C-terminal region which is necessary and sufficient for its cytoplasmic retention. Heterodimerization of RelA with either p50 or I k B a regulates its subcellular localization likely involving a dynamic mechanism contributed by the accessibility of nuclear localization sequences and nuclear export sequences to proteins within the nuclear pore complex.