MULTIALLELIC RECOGNITION - NONSELF-DEPENDENT DIMERIZATION OF THE BE AND BW HOMEODOMAIN PROTEINS IN USTILAGO-MAYDIS

In the plant pathogenic fungus Ustilago maydis, sexual and pathogenic development are controlled by the multiallelic b mating-type locus. Th e b locus encodes a pair of unrelated homeodomain proteins termed bE a nd bW, with allelic differences clustering in the N-terminal domains o f both polypeptides. Only combinations of bE and bW of different allel ic origin are active. We have investigated the underlying molecular me chanism for this intracellular self/nonself recognition phenomenon. By using the two-hybrid system, we were able to show that bE and bW dime rize only if they are derived from different alleles. Dimerization inv olves the N-terminal variable domains. Different point mutants of bE2 were isolated that function in combination with bW2. The majority of s uch bE2 mutant polypeptides were also able to form heterodimers with b W2 in the two-hybrid system. Nonself-dependent dimerization of bE and bW was supported with a biochemical interaction assay with immobilized proteins. Our results suggest a model for self/nonself recognition in which variable cohesive contacts direct dimerization.