Regulation of POU transcription factor activity by OBF1 and Sox2
For a cell to exert a specialized function certain genes have to be expressed,
others repressed. Transcription factors, regulating this expression, do not function
alone, but are often part of multi-protein complexes. Regulating a single gene with
more than one transcription factor is an efficient way to integrate responses to a
variety of signals using a limited number of proteins. DNA binding proteins often
interact with each other and with non-DNA binding proteins in a specific
arrangement. The assembly of these complexes is often highly cooperative and
promotes high levels of transcriptional synergy.
The center of my thesis is the family of POU transcription factors.
Specifically, I elaborate the interaction within the POU protein family, with members
of other transcription factor families and with cofactors. In all cases, the assembly of
the correct array of polypeptides on the DNA requires specific protein-protein and
As an example of POU factors interacting with each other and with a cofactor
I investigated the properties of a protein-DNA complex with the B-cell-specific
cofactor OBF! and the Octl dimer. Depending on the DNA sequence they bind to,
Octl dimers are arranged in configurations that are either accessible (PORE
sequence) or inaccessible (MORE sequence) to OBF!. In Chapter 3 I show that the
expression of Osteopontin, which contains a PORE sequence in its enhancer region,
depends on the presence of OBFI in B-cells. OBFI alleviates DNA sequence
requirements of the Octl dimer on PORE-related sequences in vitro. Furthermore,
OBFI enhances POU dimer-DNA interactions and overrides Oct! interface
mutations, which abolish PORE-mediated dimerization without OBFl. Based on the
biochemical data, I propose a novel Oct! dimer arrangement when OBF 1 is bound.
As an example of Oct factors interacting with members of another
transcription factor family I studied the interactions of Sox2 with Octl and Oct4,
respectively. POU and Sox transcription factors exemplify partnerships established
between various transcriptional regulators during early embryonic development. The
combination of Oct4 and Sox2 on DNA is considered to direct the establishment of
the first three lineages in the mammalian embryo.
Although functional cooperativity between key regulator proteins is pivotal
for milestone decisions in mammalian development, little is known about the
underlying molecular mechanisms. The data in Chapter 4 validate experimental highresolution
structure determination, followed by model building. The study shows that
Oct4 and Sox2 are able to dimerize on DNA in distinct conformational arrangements.
The binding site characteristics of their target genes are responsible for the correct
spatial alignment of the Velcro-like interaction domains on their surface.
Interestingly, these surfaces frequently have redundant functions and are instrumental
in recruiting various interacting protein partners.
In Chapter 5 I investigated how Sox2 and Oct4 regulate transcription of a
target gene. The first intron of Osteopontin contains a Sox-binding site and a unique
PORE to which Oct4 can either bind as a monomer or a dimer. The study reveals that
Sox2-specific repression depends on an upstream Sox site and an intact PORE,
although neither the Sox nor the PORE sites are negative elements on their own. A
mechanism is being proposed how Sox2 represses Oct4-mediated activation of