Phenotype of a Mechanosensitive Channel Mutant, mid - 1 , in a Filamentous Fungus, Neurospora crassa

Abstract

ABSTRACT In the yeast Saccharomyces cerevisiae , the MID1 ( m ating- i nduced d eath) gene encodes a stretch-activated channel which is required for successful mating; the mutant phenotype is rescued by elevated extracellular calcium. Homologs of the MID1 gene are found in fungi that are morphologically complex compared to yeast, both Basidiomycetes and Ascomycetes . We explored the phenotype of a mid - 1 knockout mutant in the filamentous ascomycete Neurospora crassa . The mutant exhibits lower growth vigor than the wild type (which is not rescued by replete calcium) and mates successfully. Thus, the role of the MID-1 protein differs from that of the homologous gene product in yeast. Hyphal cytology, growth on diverse carbon sources, turgor regulation, and circadian rhythms of the mid-1 mutant are all similar to those of the wild type. However, basal turgor is lower than wild type, as is the activity of the plasma membrane H + -ATPase (measured by cyanide [CN − ]-induced depolarization of the energy-dependent component of the membrane potential). In addition, the mutant is unable to grow at low extracellular Ca 2+ levels or when cytoplasmic Ca 2+ is elevated with the Ca 2+ ionophore A23187. We conclude that the MID-1 protein plays a role in regulation of ion transport via Ca 2+ homeostasis and signaling. In the absence of normal ion transport activity, the mutant exhibits poorer growth.