title: The amino terminus targets the mixed lineage leukemia (MLL) protein to the nucleolus, nuclear matrix and mitotic chromosomal scaffolds.
abstract: The mixed-lineage leukemia gene (MLL) is associated with more than 25 chromosomal translocations involving band 11q23 in diverse subtypes of human acute leukemia. Conditional expression of a 50 kDa amino terminal fragment spanning the AT hook motifs of MLL (MLL3AT) causes cell cycle arrest, upregulation of p21Cip1 and p27KiP1 and partial monocytic differentiation of the monoblastic U937 cell line, suggesting a major role for MLL3AT in MLL-AF9-induced myelomonocytic differentiation. In this study, we analyzed the subcellular localization of conditionally expressed MLL3AT in both U937 and HeLa cell lines. Immunofluorescence staining, confocal laser scanning microscopy and immunoelectron microscopy indicated that MLL3AT, like endogenous MLL, localized in the nucleoplasm in a punctate pattern of distribution, including regions attached to the nuclear envelope and the periphery of the nucleolus. We found that MLL3AT and endogenous MLL were present in interphase nuclear matrices and colocalized with topoisomerase II to mitotic chromosomal scaffolds. Nucleoplasm and nucleolar localization was observed even for MLL-AF9 and MLL-AF4 conditionally expressed chimeric proteins, suggesting a common target 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.
authors.affiliation: Centro di Ricerca M Tettamanti, Clinica Pediatrica Università di Milano - Bicocca, Ospedale S Gerardo, Monza, Italy.
authors.forename: C
authors.initials: C
authors.lastname: Caslini
authors.forename: A S
authors.initials: AS
authors.lastname: Alarcòn
authors.forename: J L
authors.initials: JL
authors.lastname: Hess
authors.forename: R
authors.initials: R
authors.lastname: Tanaka
authors.forename: K G
authors.initials: KG
authors.lastname: Murti
authors.forename: A
authors.initials: A
authors.lastname: Biondi
mesh_terms: D021581:Active Transport, Cell Nucleus; D020816:Amino Acid Motifs; D002453:Cell Cycle; D002466:Cell Nucleolus / Q000378:metabolism*; D002868:Chromosomal Proteins, Non-Histone / Q000378:metabolism; D022004:Chromosome Structures / Q000378:metabolism*; D004250:DNA Topoisomerases, Type II / Q000032:analysis; D004268:DNA-Binding Proteins / Q000378:metabolism* / Q000502:physiology; D019084:Fluorescent Antibody Technique, Indirect; D006367:HeLa Cells / Q000378:metabolism / Q000648:ultrastructure; D011495:Histone-Lysine N-Methyltransferase; D006801:Humans; D018613:Microscopy, Confocal; D016253:Microscopy, Immunoelectron; D008938:Mitosis; D051788:Myeloid-Lymphoid Leukemia Protein; D009363:Neoplasm Proteins / Q000378:metabolism; D019913:Nuclear Localization Signals; D015530:Nuclear Matrix / Q000378:metabolism*; D021382:Protein Sorting Signals; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D011519:Proto-Oncogenes; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D014157:Transcription Factors; D014162:Transfection; D020298:U937 Cells / Q000378:metabolism / Q000648:ultrastructure
chemical_list: D002868:Chromosomal Proteins, Non-Histone; D004268:DNA-Binding Proteins; C497422:KMT2A protein, human; D009363:Neoplasm Proteins; D019913:Nuclear Localization Signals; D021382:Protein Sorting Signals; D011993:Recombinant Fusion Proteins; D014157:Transcription Factors; D051788:Myeloid-Lymphoid Leukemia Protein; D011495:Histone-Lysine N-Methyltransferase; D004250:DNA Topoisomerases, Type II