The amino terminus targets the mixed lineage leukemia (MLL) protein to thenucleolus, nuclear matrix and mitotic chromosomal scaffolds

The mixed-lineage leukemia gene (MLL) is associated more than 25 chromosoma l translocations involving 11q23 in diverse subtypes of human acute leukemi a. Conditional expression of a 50 kDa amino terminal fragment spanning the AT hook motifs of MLL (MLL3AT) causes cell cycle arrest, upregulation of p2 1(Cip1) and p27(Kip1) and partial monocytic differentiation of the monoblas tic U937 cell line. suggesting a major role for MLL3AT in MLL-AF9-induced m yelomonocytic differentiation. In this study, we analyzed the subcellular l ocalization of conditionally expressed MLL3AT in both U937 and HeLa cell li nes. Immunofluorescence staining, confocal laser scanning microscopy and im munoelectron microscopy indicated that MLL3AT, like endogenous MLL, localiz ed in the nucleoplasm in a punctate pattern of distribution, including regi ons attached to the nuclear envelope and the periphery of the nucleolus. We found that MLL3AT and endogenous MLL were present in interphase nuclear ma trices and colocalized with topoisomerase II to mitotic chromosomal scaffol ds. Nucleoplasm and nucleolar localization was observed even for MLL-AF9 an d MLL-AF4 conditionally expressed chimeric proteins, suggesting a common ta rget conferred by the amino terminus of MLL to many if not all the chimeric MLL proteins. The nuclear matrix/scaffold association suggests a role for the amino terminus of MLL in the modulation of chromatin structure, leading to epigenetic effects on the maintenance of gene expression.