Identification and functional characterization of the novel MAR-binding protein, SATB2
The regulation of gene expression is governed in large part by transcription factors that bind to enhancers and promoters. The functions of transcription factors involve both the modulation of chromatin accessibility via the recruitment of histone-modifying enzymes or nucleosome-remodeling complexes, and the stimulation of RNA polymerase via an interaction with the mediator complex. In addition to enhancers and promoters, nuclear matrix attachment regions (MARs) have been implicated in the regulation of gene expression by altering the organization of eukaryotic chromosomes and augmenting the potential of enhancers to act over large distances. Although a lot is known about the function of MARs, the precise mechanism of their action is still obscure and probably diverse. One proposed model stipulates that their function is accomplished through the action of transcription factors, which are components of the nuclear matrix. Here, we identify and 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. In contrast to the closely related, thymocyte-specific MAR-binding protein SATB1, SATB2 is not proteolytically cleaved by caspase 6, but is instead SUMO-modified at two lysine residues. This modification is specifically augmented by the SUMO E3 ligase PIAS1. Mutation of the sumoylation sites enhances the association of SATB2 with the immunoglobulin MARs, as well as its transactivation potential. Moreover, covalent attachment of SUMO1 and SUMO3 represses SATB2- dependent transcription, without affecting either the DNA binding or the dimerization capacity of SATB2. Interestingly, SUMO conjugation affects the subnuclear localization of SATB2 and is involved in its targeting to distinct nuclear speckles (bodies). Thus, our data indicate that the regulation of SATB2 function through sumoylation can be mediated by both altering its transcriptional activation potential and by sequestering it in specific nuclear location