HETERODIMERIZATION BETWEEN 2 CLASSES OF HOMEODOMAIN PROTEINS IN THE MUSHROOM COPRINUS-CINEREUS BRINGS TOGETHER POTENTIAL DNA-BINDING AND ACTIVATION DOMAINS

The A mating type-genes of the mushroom, Coprinus cinereus, encode two classes of homeodomain-containing proteins distinguished as HD1 and H D2 on the basis of conserved, but distinctly different motifs. Compati ble mating partners bring together versions of the proteins that can h eterodimerize, thereby generating an active transcription factor compl ex that commits mated cells to sexual development. We have previously described a rare mutation in which an HD2::HD1 gene fusion generates a 'fused dimer' lacking: much of HD1 including the homeodomain yet capa ble of constitutively promoting development [Kues et al., EMBO J. 13 ( 1994b) 4054-4059]. Here, we exploit this mutation to help identify con tributions made by each protein class to normal heterodimer function. We show that the HD2 homeodomain is essential; deletion within the HD1 homeodomain can be tolerated in a normal heterodimer, as well as in t he mutant fusion protein, but not substitution of a critical amino aci d. We define, by deletion analysis, an essential C-terminal region of the HD1 and demonstrate its potential activation function by the abili ty to activate transcription in yeast when fused to the GAL4 DNA-bindi ng domain. We also identify a potential role in transcriptional repres sion for the predicted C-terminal helix of HD1 proteins.