CArG box-binding factor-A interacts with multiple motifs in immunoglobulin promoters and has a regulated subcellular distribution

CArG box-binding factor-A (CBF-A) is a protein involved in transcriptional control and interacts specifically with the penta-decameric (pd) element in K promoters. We show here that CBF-A will also bind specifically to a second region in the K promoter that overlaps with an early B cell factor binding site. Furthermore, the same region is present in multiple Ig promoters and we show that CBF-A can bind to-several of these. Mitogenic stimulation of untransformed B lymphocytes promoted nuclear localisation of CBF-A. Using enhanced GFP (EGFP)-tagged constructs and transfection into COS7 cells, a nuclear localisation signal was defined in the C terminus of CBF-A. Deletion of only 13 amino acids from the C terminus of CBF-A led to the accumulation of the protein in bright speckles at the nuclear/cytoplasmic border. We also identified the heterogeneous ribonucleoprotein H as a specific interaction partner of CBF-A, but this interaction could be detected in the cytoplasm only. Thus, CBF-A has the potential to regulate the expression of multiple Ig V genes and has a complex, mitogen-responsive regulation of its intracellular localisation.