P. Sauter et P. Matthias, COACTIVATOR OBF-1 MAKES SELECTIVE CONTACTS WITH BOTH THE POU-SPECIFICDOMAIN AND THE POU HOMEODOMAIN AND ACTS AS A MOLECULAR CLAMP ON DNA, Molecular and cellular biology (Print), 18(12), 1998, pp. 7397-7409
The lymphoid-specific transcriptional coactivator OBF-1 (also known as
OCA-B or Bob-1) is recruited to octamer site-containing promoters by
interacting with Oct-1 or Oct-2 and thereby enhances the transactivati
on potential of these two Oct factors, For this interaction the POU do
main is sufficient. By contrast, OBF-1 does not interact with the POU
domains of other POU proteins, such as Oct-4, Oct-6, or Pit-1, even th
ough these factors bind efficiently to the octamer motif, Here we exam
ined the structural requirements for selective interaction between the
POU domain and OBF-1, Previous data have shown that formation of a te
rnary complex among OBF-1, the POU domain, and the DNA is critically d
ependent on residues within the octamer site, By methylation interfere
nce analysis we identified bases that react differently in the presenc
e of OBF-1 compared to the POU domain alone, and using phosphothioate
backbone-modified probes in electrophoretic mobility shift assays, we
identified several positions influencing ternary complex formation, We
then used Oct-1/Pit-1 POU domain chimeras to analyze the selectivity
of the interaction between OBF-1 and the POU domain, This analysis ind
icated that both the POU specific domain (POU,) and the POU homeodomai
n (POU,) contribute to complex formation. Amino acids that are differe
nt in the Pit-1 and Oct-1 POU domains and are considered to be solvent
accessible based on the Oct-1 POU domain/DNA cocrystal structure were
replaced,vith alanine residues and analyzed for their influence on co
mplex formation, Thereby, we identified residues L6 and E7 in the POUs
and residues K155 and 1159 in the POUH to be critical in vitro and in
vivo for selective interaction with OBF-1, Furthermore, in an in vivo
assay we could show that OBF-1 is able to functionally recruit two ar
tificially separated halves of the POU domain to the promoter DNA, the
reby leading to transactivation, These data allow us to propose a mode
l of the interaction between OBF-1 and the POU domain, whereby OBF-1 a
cts as a molecular clamp holding together the two moieties of the POU
domain and the DNA.