SHORT ISOFORM OF POU FACTOR BRN-3B CAN FORM A HETERODIMER WITH BRN-3ATHAT IS INACTIVE FOR OCTAMER MOTIF BINDING

The POU proteins Brn-3a and Brn-3b belong to a family of DNA binding t ranscription factors that share stretches of extensive homology. Both Brn-3a and Brn-3b are expressed as shorter and longer isoforms. The lo ng form of Brn-3a is able to oncogenically transform primary fibroblas ts. By contrast, the short form of Brn-3b (Brn-3b(s)) cannot transform fibroblasts but is able to specifically inhibit the transforming acti vity of Brn-3a(l). Moreover, Brn-3a(l) can act as a transcriptional tr ansactivator, while Brn-3b(s) is not only unable to do so but in addit ion specifically inhibits the transactivating activity of Brn-3a(l). H ere, we show that the opposite and antagonistic activities of Brn-3a(l ) and Brn-3b(s) proteins are due to their different DNA binding proper ties; Brn-3a(l) but not Brn-3b(s) can form stable complexes with sever al octamer-related target DNA sequences. The presence of Brn-3b(s) com pletely inhibits the binding of Brn-3a(l) to DNA by preventing the for mation of Brn-3a(l)-DNA complexes as well as by disrupting preformed c omplexes. Experiments with GST fusion proteins and in vitro binding st udies suggest that the inhibition of Brn-3a(l) activity by Brn-3b(s) o ccurs via direct interaction of the two transcription factors in solut ion. Therefore, we hypothesize that Brn-3b(s) can act as a direct anta gonist of Brn-3a(l) by inhibiting its DNA binding through the formatio n of an inactive hetero-oligomeric complex.