The human T-cell leukemia virus type I Tax protein transforms T cells through 
induced expression of many cellular genes, including those encoding the 
growth-related proteins interleukin 2 and the alpha chain of its receptor. 
Induction of these genes is mediated, at least in part, through Tax-dependent 
posttranslational activation of NF-kappa B, typically heterodimers of p50 
(NF-kappa B1) and p65 (RelA). The preexisting NF-kappa B proteins are retained 
in the cytoplasm of cells by association with inhibitory 
ankyrin-motif-containing I kappa B proteins, primarily I kappa B-alpha but also 
including the precursor proteins p105 (NF-kappa B1) and p100 (NF-kappa B2). Here 
we demonstrate the existence of a previously undescribed multimeric cytoplasmic 
complex in which NF-kappa B dimers are associated with the p100 inhibitor in a 
manner dependent on the precursor protein's ankyrin domain. We also demonstrate 
an antagonistic effect of the Tax protein on the cytoplasmic sequestration 
function of p100; this in turn leads to nuclear translocation of NF-kappa B 
dimers liberated from multimeric complexes. Tax may exert these effects through 
the physical association with p100. Tax also relieves the p100-mediated 
inhibition of DNA binding by p50-p65 heterodimers in vitro. The results 
demonstrate a mechanism by which Tax may activate NF-kappa B in T cells.