Ultrastructure of the Aspergillus nidulans hypA1 restrictive phenotype shows defects in endomembrane arrays and polarized wall deposition

Aspergillus nidulans Eidam (G. Wint.) wild-type hyphal morphogenesis requires the hypA gene product. Like its homolog in Saccharomyces cerevisiae Meyen ex E.C. Hansen, TRS120, hypA encodes a cytoplasmic protein likely associated with endomem branes. hypA is not essential, but hypA1 temperature-sensitive strains grow poorly at restrictive temperatures. In younger cells, endomembrane arrays were aberrant, only sometimes resembling wild type. In older cells, Golgi equivalents were swollen, impacted with electron-dense granules. In hypA1 strains grown at 42 °C, the poorly polarized hyphae lack recognizable Spitzenkörper and have walls at least four-fold thicker than those of wild-type or hypA1 strains grown at 28 °C. At restrictive temperatures, both hyphal width and wall thickness increase markedly in basal regions, suggesting wall deposition is impaired. Septa are thicker than in wild type, but have medial pores and Woronin bodies. Individual nuclei and mitochondria are smaller at 42 °C than at 28 °C, but each collectively occupies similar proportions of the cytoplasm. Mitochondrial cristae are reduced in number and width at 42 °C, possibly compromising metabolic efficiency; in older cells, cristae are widely spaced and randomly inserted. If hypA1 cells grown at 42 °C are shifted to 28 °C, the thickened wall is precisely degraded for growth of wild-type branches, which form within 1 h, suggesting areas of nascent polarity formed at 42 °C require the hypA product for wild-type function.Key words: endomembrane, filamentous fungus, electron microscopy, cell wall, secretion, Saccharomyces TRS120.