Phenotypic abnormalities of fr, sp, and och-1 single mutants are suppressed by loss of putative GPI-phospholipase A2 in Neurospora crassa

Abstract

Two Neurospora crassa mutants—frost (fr), an orthologous gene of Saccharomyces cerevisiae CDC1 that encodes a putative mannose-ethanolamine phosphate phosphodiesterase involved in glycosyl-phosphatidylinositol (GPI)-anchor remodeling, and spray (sp), a filamentous fungus-specific gene that encodes a putative Ca2+ channel protein—have been known to exhibit reduced hyphal growth and hyperbranched mycelial phenotypes. However, they are suppressed by exogenous Ca2+. We characterized a N. crassa deletion mutant of och-1, which encodes an α-1,6-mannosyltransferase required for synthesis of galactomannans attached to N-linked oligosaccharides of cell wall proteins, and found that the mutant showed growth defects quite similar to those of fr and sp single mutants. The abnormal morphologies of the och-1 mutant were also significantly restored by exogenous Ca2+, suggesting the contribution of Ca2+ in cell wall stress response. Because abnormalities observed in S. cerevisiae CDC1 mutants have been shown to be suppressed by deletion of PER1 that encodes GPI-phospholipase A2, we also characterized a N. crassa strain lacking its sole PER1 ortholog, gpl-1. We found that this strain exhibited growth retardation but not hyperbranching of mycelia. Deletion of gpl-1 in the fr mutant suppressed growth defects seen in the single mutant, just as the PER1 deletion suppressed those in the CDC1 mutant of S. cerevisiae. The deletion of gpl-1 also suppressed defects in N. crassa sp and och-1 single mutants. These results suggest the presence of a genetic interaction(s) between calcium signaling pathways and lipid remodeling processes of GPI-anchored proteins in N. crassa.