Plasma Membrane Fusion Is Specifically Impacted by the Molecular Structure of Membrane Sterols During Vegetative Development of Neurospora crassa

Abstract

Cell-cell fusion is crucial for the development and propagation of most eukaryotic organisms. Despite this importance, the molecular mechanisms mediating this process are only poorly understood in any biological system. In particular, the step of plasma membrane merger and the contributing proteins and physicochemical factors remain mostly unknown. Earlier studies provided first evidence of a role of membrane sterols in cell-cell fusion. By characterizing different ergosterol biosynthesis mutants of the fungus Neurospora crassa, which accumulate different ergosterol precursors, we show that the structure of the sterol ring system specifically affects plasma membrane merger during the fusion of vegetative spore germlings. Genetic analyses pinpoint this defect to an event prior to engagement of the fusion machinery. Strikingly, this effect is not observed during sexual fusion, suggesting that the specific sterol precursors do not generally block membrane merger, but rather impair subcellular processes exclusively mediating fusion of vegetative cells. On a colony-wide level, the altered structure of the sterol ring system affects a subset of differentiation processes, including vegetative sporulation and steps before and after fertilization during sexual propagation. Together these observations corroborate the notion that the accumulation of particular sterol precursors has very specific effects on defined cellular processes rather than non-specifically disturbing membrane functioning. Given the phenotypic similarities of the ergosterol biosynthesis mutants of N. crassa during vegetative fusion and of Saccharomyces cerevisiae cells undergoing mating, our data support the idea that yeast mating is evolutionary and mechanistically more closely related to vegetative than sexual fusion of filamentous fungi.

A focus on yeast mating: From pheromone signaling to cell-cell fusion

“…The absence of the ergosterol biosynthetic enzymes Erg4 and Erg6 leads to fusion defects in S. cerevisiae, likely due to the accumulation of precursors rather than absence of ergosterol [148,150]. Indeed, a recent study in N. crassa links the structure of ergosterol precursors with membrane fusion defects [151].…”

Analysis of the Putative Nucleoporin POM33 in the Filamentous Fungus Sordaria macrospora

“…Besides, several S. macrospora homologs of the ergosterol-biosynthesis pathway were also enriched in the pulldown experiments, namely ERG6 (SMAC_01214), ERG11 (SMAC_05737 or SMAC_05894), ERG1 (SMAC_00934), ERG26 (SMAC_04350), ERG25 (SMAC_00952) and ERG9 (SMAC_07413). Of interest, in N. crassa the knockout of erg2 or erg1 leads to cell fusion defects of hyphae [ 98 , 99 ]. This phenotype of dysfunctional hyphal fusion was also described for mutants of SmSTRIPAK components like in Δsci1 [ 14 ].…”

RNA-dependent synthesis of ergosteryl-3β-O-glycine in Ascomycota expands the diversity of steryl-amino acids

“…Like cholesterol (Cho) in mammalian cells, it influences the activity of membrane proteins, including V-ATPases that participate in antifungal drugs resistance (37,38), participates in the formation of membrane microdomains (39,40) and, together with sphingolipids, regulate membrane trafficking and polarized cell growth in filamentous fungi (41,42). Even the mating process, during sexual reproduction, is dependent on the integrity of Erg for membranes fusion (43,44) or for pheromone sensing (45). Erg also plays multiple roles in pathogenicity and antifungal resistance (10,46,47).…”