A ROLE FOR HETERODIMERIZATION IN NUCLEAR-LOCALIZATION OF A HOMEODOMAIN PROTEIN

The A mating type genes of the mushroom Coprinus cinereus encode two f amilies of dissimilar homeodomain proteins (HD1 and HD2). The proteins heterodimerize when mating cells fuse to generate a transcriptional r egulator that promotes expression of genes required for early steps in sexual development. In previous work we showed that heterodimerizatio n brings together different functional domains of the HD1 and HD2 prot eins; a potential activation domain at the C terminus of the HD1 prote in and an essential HD2 DNA-binding motif. Two predicted nuclear local ization signals (NLS) are present in the HD1 protein but none are in t he HD2 protein. We deleted each NLS separately from an HD1 protein and showed that one (NLS1) is essential for normal heterodimer function. Fusion of the NLS sequences to the C terminus of an HD2 protein compen sated for their deletion from the HD1 protein partner and permitted th e two modified proteins to form a functional transcriptional regulator . The nuclear targeting properties of the A protein NLS sequences were demonstrated by fusing the region that encodes them to the bacterial uidA (beta-glucuronidase) gene and showing that beta-glucuronidase exp ression localized to the nuclei of onion epidermal cells. These observ ations lead to the proposal that heterodimerization regulates entry of the active transcription factor complex to the nucleus.