BCL11A-XL protein accumulates in the nuclear matrix of pre-germinal center lymphomas and B cell lines. (A) BCL11A-XL, whether endogenous (left panel) or ectopically expressed as an N-terminal FLAG-tagged fusion in HEK293 cells (right panel) partitions predominantly into the nuclear matrix fraction. Cells were fractionated using the method of Reyes et al , and equivalent volumes of each fraction were loaded per lane, allowing us to assess the relative distribution by western blotting with BCL11A/123. Cellular compartments were fractionated as Cytoplasm (C), Nucleoplasm (NP), Chromatin (CH), and Nuclear Matrix (NM). BCL11A-L partitions predominantly into the chromatin-associated fraction. BCL11A-S is predominantly cytoplasmic (data not shown). Note that HEK293 cells express endogenous XL protein in the NM fraction. (B) Endogenous BCL6, like BCL11A-XL, accumulates within the nuclear matrix. (C) Conservation of the XL isoform is documented by Western blotting a murine B-cell line (BCL1), mouse whole brain tissue, and the chicken DT40 B-cell line.