Role of a Mitogen-Activated Protein Kinase Pathway during Conidial Germination and Hyphal Fusion in Neurospora crassa

Apr 13, 2004Eukaryotic cell

Role of a cell signaling pathway in spore germination and fungal cell fusion in Neurospora crassa

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Abstract

A mutant of the filamentous fungus Neurospora crassa lacking the mak-2 gene exhibits multiple developmental defects.

The mak-2 mutant shows derepressed conidiation, shortened aerial hyphae, and lack of vegetative hyphal fusion.
Female sterility and autonomous ascospore lethality are also observed in the mak-2 mutant.
Phosphorylation levels of MAK-2 are highest during germ tube elongation, branching, and hyphal fusion.
A predicted MAP kinase kinase kinase, NRC-1, is homologous to STE11 in Saccharomyces cerevisiae.
The nrc-1 mutant shares phenotypic traits with the mak-2 mutant, particularly hyphal fusion defects.
MAK-2 phosphorylation during early colony development may depend on NRC-1.

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Mitogen-activated protein (MAP) kinase signaling pathways are ubiquitous and evolutionarily conserved in eukaryotic organisms. MAP kinase pathways are composed of a MAP kinase, a MAP kinase kinase, and a MAP kinase kinase kinase; activation is regulated by sequential phosphorylation. Components of three MAP kinase pathways have been identified by genome sequence analysis in the filamentous fungus Neurospora crassa. One of the predicted MAP kinases in N. crassa, MAK-2, shows similarity to Fus3p and Kss1p of Saccharomyces cerevisiae, which are involved in sexual reproduction and filamentation, respectively. In this study, we show that an N. crassa mutant disrupted in mak-2 exhibits a pleiotropic phenotype: derepressed conidiation, shortened aerial hyphae, lack of vegetative hyphal fusion, female sterility, and autonomous ascospore lethality. We assessed the phosphorylation of MAK-2 during conidial germination and early colony development. Peak levels of MAK-2 phosphorylation were most closely associated with germ tube elongation, branching, and hyphal fusion events between conidial germlings. A MAP kinase kinase kinase (NRC-1) is the predicted product of N. crassa nrc-1 locus and is a homologue of STE11 in S. cerevisiae. An nrc-1 mutant shares many of the same phenotypic traits as the mak-2 mutant and, in particular, is a hyphal fusion mutant. We show that MAK-2 phosphorylation during early colony development is dependent upon the presence of NRC-1 and postulate that phosphorylation of MAK-2 is required for hyphal fusion events that occur during conidial germination.

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Role of a Mitogen-Activated Protein Kinase Pathway during Conidial Germination and Hyphal Fusion in Neurospora crassa

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