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

Abstract

ABSTRACT 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.