Interaction of the A alpha Y and Z mating-type homeodomain proteins of Schizophyllum commune detected by the two-hybrid system.

The A alpha locus is one of four mating-type loci that control sexual development in Schizophyllum commune. A alpha has nine alternative mating types (A alpha 1-A alpha 9) which encode specific alleles of two homeodomain-related proteins, Y and Z. For example, proteins Y4 and Z4 are encoded by A alpha 4, and Y5 and Z5 are encoded by A alpha 5. Our previous studies showed that A alpha-regulated development is activated in fusion cells between haploid strains by the presence of Y and Z proteins derived from different A alpha mating types (e.g., Y4 and Z5), but not by proteins from the same mating type (e.g., Y4 and Z4). In this study we analyzed protein-protein interactions of Y4 with the Z4 and Z5 isoforms in a two-hybrid system. Protein interactions were detected in filter and liquid assays for beta-galactosidase with Y4 and Z5, a non-self, developmentally active combination, but not with Y4 and Z4, a self, developmentally inactive combination. These results support the hypothesis that non-self combinations of Y and Z form a heteromultimer that activates development, but that self combinations do not. Although developmental target genes directly regulated by the A alpha locus have yet to be identified, it is assumed on the basis of motifs encoded that the Y-Z heteromultimer acts as a transcription factor.