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 thr ee or more paralogous pairs of genes encoding two families of homeodom ain proteins (HD1 and HD2). A successful mating brings together differ ent allelic forms of at least one gene, and this is sufficient to trig ger initial steps in sexual development. Previous studies have suggest ed that development is regulated by heterodimerization between HD1 and HD2 proteins, In this report, we describe 5' gene deletions and 5' en d exchanges showing that the N-terminal regions of the proteins are es sential 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 protein s and found that heterodimerization only occurs between compatible pro tein combinations. The N-terminal regions of the proteins were suffici ent to mediate dimerization, and N-terminal swaps resulted in a predic ted change in dimerization specificity. By analyzing the N-terminal am ino 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.