ASPERGILLUS-NIDULANS APSA (ANUCLEATE PRIMARY STERIGMATA) ENCODES A COILED-COIL PROTEIN REQUIRED FOR NUCLEAR POSITIONING AND COMPLETION OF ASEXUAL DEVELOPMENT

Many fungi are capable of growing by polarized cellular extension to f orm hyphae or by isotropic expansion to form buds. Aspergillus nidulan s anucleate primary sterigmata (apsA) mutants are defective in nuclear distribution in both hyphae and in specialized, multicellular reprodu ctive structures, called conidiophores. apsA mutations have a negligib le effect on hyphal growth, unlike another class of nuclear distributi on (nud) mutants. By contrast, they almost completely block entry of n uclei into primary buds, or sterigmata (bud nucleation), produced duri ng development of conidiophores. Failure of the primary sterigmata to become nucleated results in developmental ar rest and a failure to act ivate the transcriptional program associated with downstream developme ntal steps. However, occasionally in mutants a nucleus enters a primar y bud and this event relieves the developmental blockage. Thus, there is a stringent developmental requirement for apsA function, but only a t the stage of primary bud formation. apsA encodes a 183-kD coiled-coi l protein with similarity to Saccharomyces cerevisiae NUM1p, required for nuclear migration in the budding process.