2 CLASSES OF HOMEODOMAIN PROTEINS SPECIFY THE MULTIPLE-A MATING TYPESOF THE MUSHROOM COPRINUS-CINEREUS

The A mating type locus of the mushroom Coprinus cinereus regulates es sential steps in sexual development. The locus is complex and contains several functionally redundant, multiallelic genes that encode putati ve transcription factors. Here, we compare four genes from an A locus designated A42. Overall, the DNA sequences are very different (similar to 50% homology), but two classes of genes can be distinguished on th e basis of a conserved homeodomain motif in their predicted proteins ( HD1 and HD2). Development is postulated to be triggered by an HD1 and an HD2 gene from different A loci. Thus, proteins encoded by genes of the same locus must be distinguished from those encoded by another loc us. Individual proteins of both classes recognize each other using the region N-terminal to the homeodomain. These N-terminal specificity re gions (COP1 and COP2) are predicted to be helical and are potential di merization interfaces. The amino acid composition of the C-terminal re gions of HD1 proteins suggests a role in activation, and gene truncati ons indicate that this region is essential for function in vivo. A cor responding C-terminal region in HD2 proteins can be dispensed with in vivo. We will discuss these predicted structural features of the C. ci nereus A proteins, their proposed interactions following a compatible cell fusion, and their similarities to the a1 and alpha 2 mating type proteins of the yeast Saccharomyces cerevisiae.