Expression of I kappa B alpha in the nucleus of human peripheral blood T lymphocytes

According to current models the inhibitory capacity of I kappa B alpha woul d be mediated through the retention of Rel/NF-kappa B proteins in the cytos ol. However, I kappa B alpha has also been detected in the nucleus of cell lines and when overexpressed by transient transfection. To gain better insi ght into the potential role of nuclear I kappa B alpha in a physiological c ontext we have analysed its presence in the nucleus of human peripheral blo od T lymphocytes (PBL). We demonstrate the nuclear localization of I kappa B alpha in PBL by different techniques: Western blot, indirect immunofluore scence and electron microscopy. Low levels of nuclear I kappa B alpha were detected in resting cells whereas a superinduction was obtained after PMA a ctivation. The nuclear pool of I kappa B alpha showed a higher stability th an cytosolic I kappa B alpha and was partially independent of the resynthes is of the protein. Unexpectedly, the presence of nuclear I kappa B alpha di d not inhibit NF-kappa B binding to DNA and this phenomenon was not due to the presence of I kappa B beta at the nuclear level. Immunoprecipitation ex periments failed to demonstrate an association between nuclear I kappa B al pha and NF-kappa B proteins. Our results demonstrate that in resting and PM A-activated human PBL, I kappa B alpha is present in the nucleus in an appa rently inactive form unable to disrupt NF-kappa B binding from DNA.