Identification of a novel NF-kappaB p50-related protein in B lymphocytes

In most cell types other than mature B lymphocytes and macrophages, the transcription factor NF-kappaB remains in an inactive form in the cytosol by being bound to the inhibitory proteins IkappaBalpha and IkappaBbeta. To investigate the regulation of constitutively active NF-kappaB in B lymphocytes, we have examined the composition of Rel protein complexes in different mouse B-cell lines. As reported previously, the constitutively active complex in mature B cells was predominantly p50:c-Rel. However, the kappaB binding complex in the plasmacytomas that were examined lacked c-Rel and instead contained only a p50-related protein. This p50-related protein (p55) cross-reacts with three different p50 antisera, exists in both the cytosol and the nucleus, and is the protein that binds to kappaB sites in plasma cells. Transfection of reporter constructs into plasma cells indicates that the p55 complex is also transcriptionally active. The p55 protein can be detected in splenocytes from mice lacking the p105/p50 gene, and therefore it appears to be the product of a distinct gene. The implications of the existence of a NF-kappaB p50-related protein in plasma cells that is capable of binding to kappaB sites and activating transcription are discussed.