Intramolecular control of transcriptional activity by the NK2-specific domain in NK-2 homeodomain proteins

The developmentally important homeodomain transcription factors of the NK-2 class contain a highly conserved region, the NK2-specific domain (NK2-SD), The function of this domain, however, remains unknown. The primary structu re of the NK2-SD suggests that it might function as an accessory DNA-bindin g domain or as a protein-protein interaction interface. To assess the possi bility that the NK2-SD may contribute to DNA-binding specificity, we used a PCR-based approach to identify a consensus DNA-binding sequences for Nkx2. 2, an NK-2 family member involved in pancreas and central nervous system de velopment. The consensus sequence (T(C)(T)AAGT(GG)(AC)TT) is similar to the known binding sequences for other NK-2 homeodomain proteins, but we show t hat the NK2-SD does not contribute significantly to specific DNA binding to this sequence. To determine whether the NK2-SD contributes to transactivat ion, we used GAL4-Nkx2.2 fusion constructs to map a powerful transcriptiona l activation domain in the C-terminal region beyond the conserved NK2-SD. i nterestingly, this C-terminal region functions as a transcriptional activat or only in the absence of an intact NK2-SD, The NK2-SD also can mask transa ctivation from the paired homeodomain transcription factor Pax6, but it has no effect on transcription by itself, These results demonstrate that the N K2-SD functions as an intramolecular regulator of the C-terminal activation domain in Nkx2.2 and support a model in which interactions through the NK2 -SD regulate the ability of NK-2-class proteins to activate specific genes during development.