Phenotype of a mechanosensitive channel mutant, mid-1, in a filamentous fungus, Neurospora crassa

In the yeast Saccharomyces cerevisiae, the MID1 (mating-induced death) 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 Ca2+ levels or when cytoplasmic Ca2+ is elevated with the Ca2+ ionophore A23187. We conclude that the MID-1 protein plays a role in regulation of ion transport via Ca2+ homeostasis and signaling. In the absence of normal ion transport activity, the mutant exhibits poorer growth.