SATB1 regulates β-like globin genes through matrix related nuclear relocation of the cluster

The nuclear location and relocation of genes play crucial regulatory roles in gene expression. SATB1, a MAR-binding protein, has been found to regulate β-like globin genes through chromatin remodeling. In this study, we generated K562 cells over-expressing wild-type or nuclear matrix targeting sequences (NMTS)-deficient SATB1 and found that like wild-type SATB1, NMTS-deficient SATB1 induces out loop of β-globin cluster from its chromosome territory (CT), while it is unable to associate the cluster with the nuclear matrix as wild-type SATB1 does and had no regulatory functions to the β-globin cluster. Besides, our data showed that the transacting factor occupancies and chromatin modifications at β-globin cluster were differentially affected by wild-type and NMTS-deficient SATB1. These results indicate that SATB1 regulates β-like globin genes at the nuclear level interlaced with chromatin and DNA level, and emphasize the nuclear matrix binding activity of SATB1 to its regulatory function.     

The behavior of SATB1, a MAR-binding protein, in response to apoptosis stimulation

As a MAR-binding protein, SATB1 regulates genes by folding chromatin into a loop domain. Apoptosis is known to be accompanied by a collapse of nuclear architecture and cleavage of condensing chromatin into oligonucleosomal fragments. To further understand the functional role of MAR-binding proteins during apoptosis we investigated the relationship of the behavior of SATB1 and the collapse of nuclear architecture in Jurkat cells with immunostaining and Western blot analysis. We demonstrated that SATB1 formed special three-dimensional network distributions during early apoptosis. The distribution change of SATB1 was associated with cleavage of the protein and accompanied by the nuclear architecture collapse. Cleavage of SATB1 was mediated by caspase-3 and was apoptosis specific. Our observations further support the notion that early proteolysis of MAR-binding proteins might represent a universal mechanism that renders these DNA sites vulnerable to endonucleolysis.     

The role of nuclear matrix proteins binding to matrix attachment regions (Mars) in prostate cancer cell differentiation

In tumor progression definite alterations in nuclear matrix (NM) protein composition as well as in chromatin structure occur. The NM interacts with chromatin via specialized DNA sequences called matrix attachment regions (MARs). In the present study, using a proteomic approach along with a two-dimensional Southwestern assay and confocal laser microscopy, we show that the differentiation of stabilized human prostate carcinoma cells is marked out by modifications both NM protein composition and bond between NM proteins and MARs. Well-differentiated androgen-responsive and slowly growing LNCaP cells are characterized by a less complex pattern and by a major number of proteins binding MAR sequences in comparison to 22Rv1 cells expressing androgen receptor but androgen-independent. Finally, in the poorly differentiated and strongly aggressive androgen-independent PC3 cells the complexity of NM pattern further increases and a minor number of proteins bind the MARs. Furthermore, in this cell line with respect to LNCaP cells, these changes are synchronous with modifications in both the nuclear distribution of the MAR sequences and in the average loop dimensions that significantly increase. Although the expression of many NM proteins changes during dedifferentiation, only a very limited group of MAR-binding proteins seem to play a key role in this process. Variations in the expression of poly (ADP-ribose) polymerase (PARP) and special AT-rich sequence-binding protein-1 (SATB1) along with an increase in the phosphorylation of lamin B represent changes that might trigger passage towards a more aggressive phenotype. These results suggest that elucidating the MAR-binding proteins that are involved in the differentiation of prostate cancer cells could be an important tool to improve our understanding of this carcinogenesis process, and they could also be novel targets for prostate cancer therapy.     

Structural basis for recognition of the matrix attachment region of DNA by transcription factor SATB1

Special AT-rich sequence binding protein 1 (SATB1) regulates gene expression essential in immune T-cell maturation and switching of fetal globin species, by binding to matrix attachment regions (MARs) of DNA and inducing a local chromatin remodeling. Previously we have revealed a five-helix structure of the N-terminal CUT domain, which is essentially the folded region in the MAR-binding domain, of human SATB1 by NMR. Here we determined crystal structure of the complex of the CUT domain and a MAR DNA, in which the third helix of the CUT domain deeply enters the major groove of DNA in the B-form. Bases of 5'-CTAATA-3' sequence are contacted by this helix, through direct and water-mediated hydrogen bonds and apolar and van der Waals contacts. Mutations at conserved base-contacting residues, Gln402 and Gly403, reduced the DNA-binding activity, which confirmed the importance of the observed interactions involving these residues. A significant number of equivalent contacts are observed also for typically four-helix POU-specific domains of POU-homologous proteins, indicating that these domains share a common framework of the DNA-binding mode, recognizing partially similar DNA sequences.     

Binding of matrix attachment regions to lamin B1.

Eukaryotic chromatin is organized into topologically constrained loops that are attached to the nuclear matrix. The regions of DNA that interact with the matrix are called matrix attachment regions (MARs). We studied the spatial distribution of MAR-binding sites in the nuclear matrix from rat liver cells, following a combined biochemical and ultrastructural approach. We found that MAR-binding sites are distributed equally over the internal fibrogranular network and the peripheral nuclear lamina. Internal and peripheral binding sites have similar binding characteristics: both sets of binding sites show specific and saturable binding of MARs from different organisms. By means of a DNA-binding protein blot assay and in vitro binding studies, we identified lamin B1 as a MAR-binding protein, which provides evidence for a specific interaction of DNA with the nuclear lamina.