SATB1

Abstract: Nuclear matrix attachment regions (MARs) are regulatory DNA sequences that are important for higher-order chromatin organization, long-range enhancer function, and extension of chromatin modifications. Here we characterize a novel cell type-specific MAR-binding protein, SATB2, which binds to the MARs of the endogenous immunoglobulin μ locus in pre-B cells and enhances gene expression. We found that SATB2 differs from the closely related thymocyte-specific protein SATB1 by modifications of two lysines with the small ubiquitive related modifier (SUMO), which are augmented specifically by the SUMO E3 ligase PIAS1. Mutations of the SUMO conjugation sites of SATB2 enhance its activation potential and association with endogenous MARs in vivo, whereas N-terminal fusions with SUMO1 or SUMO3 decrease SATB2-mediated gene activation. Sumoylation is also involved in targeting SATB2 to the nuclear periphery, raising the possibility that this reversible modification of a MAR-binding protein may contribute to the modulation of subnuclear DNA localization.

Abstract: Satb1 is a first cell-type-specific transcription factor of a novel type that functions as a regulator of the transcription of large chromatin domains. We identified a close homologue of Satb1, Satb2, in a cDNA subtraction screening in a search for genes controlling neural differentiation. Satb2 showed 61% amino-acid homology to Satb1. Satb2 and Satb1 expression was detected in different cell subpopulations of developing mouse CNS in a mutually exclusive manner. In the electrophoretic mobility shift assay we demonstrate that nuclear extracts from the embryonic day 18.5 mouse developing neocortex, in contrast to basal ganglia, contain a protein complex interacting with matrix attachment region DNA elements (MARs) with high affinity. Endogenous Satb2 protein is a part of this complex. In the developing neocortex Satb2 was detected largely in the superficial layers. In the developing spinal cord Satb2 expression marks a subpopulation of Lbx1-positive neurons dorsally and a subgroup of Isl1-positive neurons ventrally. In the Lbx1 mutants Satb2 expression is greatly reduced. We suggest that Satb2 may regulate differentiation of subsets of neurons at the level of higher order chromatin structure via binding to MARs.