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