THE SPECIFICITY DETERMINANT OF THE Y MATING-TYPE PROTEINS OF SCHIZOPHYLLUM-COMMUNE IS ALSO ESSENTIAL FOR Y-Z PROTEIN-BINDING

This paper concerns the manner in which combinatorial mating proteins of the fungus, Schizophyllum commune, recognize one another to form co mplexes that regulate target gene expression. In Schizophyllum, tightl y linked Y and Z mating-type genes do not promote development in the c ombinations present in haploid strains (i.e., self combinations). When the Y and Z genes from two different mating types are brought togethe r by the fusion of two haploid cells, the Y and Z proteins from differ ent mating types recognize one another as nonself; form a complex and activate development. Several Y and Z alleles are present in the popul ation and all nonself combinations of Y and Z alleles are equally func tional. We have made chimeric genes among Y1, Y3, Y4 and M and examine d their mating-type specificities by transformation and mating tests. These studies show that the specificity of Y protein recognized by Z p rotein is encoded within a short region of N-terminal amino acids. The critical region is not precisely the same in each Y protein and in ea ch Y-Z protein interaction. For Y3 protein compared with Y4 protein, t he critical residues are in an N-terminal region of 56 amino acids (re sidues 17-72), with 40% identity and 65% similarity. Two-hybrid studie s show that: the first 144 amino acids of Y4 protein are sufficient to bind Z3 and Z5 proteins, but not Z4 protein, and proteins deleted of the Y4 specificity region do not bind Z3, Z4 or Z5 protein. Thus the s pecificity determinant of the Y protein is essential for protein-prote in recognition, Y-Z protein binding and mating activity.