Heterodimerization between two 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 HD2 on the basis of conserved, but distinctly different motifs. Compatible mating partners bring together versions of the proteins that can heterodimerize, thereby generating an active transcription factor complex 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 capable of constitutively promoting development [Kües et al., EMBO J. 13 (1994b) 4054-4059]. Here, we exploit this mutation to help identify contributions 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 the mutant fusion protein, but not substitution of a critical amino acid. We define, by deletion analysis, an essential C-terminal region of the HD1 and demonstrate its potential activation function by the ability to activate transcription in yeast when fused to the GAL4 DNA-binding domain. We also identify a potential role in transcriptional repression for the predicted C-terminal helix of HD1 proteins.

An N-Terminal Dimerization Domain Permits Homeodomain Proteins To Choose Compatible Partners and Initiate Sexual Development in the Mushroom Coprinus cinereus.

The A mating-type locus of the mushroom Coprinus cinereus contains three or more paralogous pairs of genes encoding two families of homeodomain proteins (HD1 and HD2). A successful mating brings together different allelic forms of at least one gene, and this is sufficient to trigger initial steps in sexual development. Previous studies have suggested that development is regulated by heterodimerization between HD1 and HD2 proteins. In this report, we describe 5[prime] gene deletions and 5[prime] end exchanges showing that the N-terminal regions of the proteins are essential for choosing a compatible partner but not for regulating gene transcription. Using an in vitro glutathione S-transferase association assay, we demonstrated heterodimerization between HD1 and HD2 proteins and found that heterodimerization only occurs between compatible protein combinations. The N-terminal regions of the proteins were sufficient to mediate dimerization, and N-terminal swaps resulted in a predicted change in dimerization specificity. By analyzing the N-terminal amino acid sequences of HD1 proteins, we identified two potential coiled-coil motifs whose relative positions vary in paralogous proteins but are both required for in vivo function.

Two classes of homeodomain proteins specify the multiple a mating types of the mushroom Coprinus cinereus.

The A mating type locus of the mushroom Coprinus cinereus regulates essential steps in sexual development. The locus is complex and contains several functionally redundant, multiallelic genes that encode putative transcription factors. Here, we compare four genes from an A locus designated A42. Overall, the DNA sequences are very different (approximately 50% homology), but two classes of genes can be distinguished on the 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 locus. Individual proteins of both classes recognize each other using the region N-terminal to the homeodomain. These N-terminal specificity regions (COP1 and COP2) are predicted to be helical and are potential dimerization interfaces. The amino acid composition of the C-terminal regions of HD1 proteins suggests a role in activation, and gene truncations indicate that this region is essential for function in vivo. A corresponding C-terminal region in HD2 proteins can be dispensed with in vivo. We will discuss these predicted structural features of the C. cinereus 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.