THE CLASS-III POU FACTOR BRN-4 INTERACTS WITH OTHER CLASS-III POU FACTORS AND THE HETEROGENEOUS NUCLEAR RIBONUCLEOPROTEIN-U

The class III POU proteins are expressed throughout the central nervou s system including the hypothalamus, where they are often co-localized . Presumably, these POU proteins (Brain-1, Brain-2, Brain-4 and SCIP) serve as transcriptional transactivators. That they are co-expressed i n some neurons suggests that, if they were to form homomeric and heter omeric complexes with each other, depending on the particular combinat ion, they might have different DNA-binding specificities and, thus, ac tivate different genes. We used purified fusion proteins of the four c lass III POU proteins in far-western assays to show that the proteins can interact. We confirmed their interactions using a two-hybrid syste m. Both techniques indicate that the interaction occurs through the PO U domain. The far-western technique also allowed us to identify a 120- kDa nuclear protein that interacts with Brain-4. Subsequent affinity p urification and microsequencing identified the protein as the heteroge neous nuclear ribonucleoprotein U (hnRNP U). This result suggests anot her mechanism by which a POU protein can influence gene expression: by facilitating the processing of pre-mRNA whose transcription it has st imulated.