Mitogen-Activated Protein Kinase Cascade Required for Regulation of Development and Secondary Metabolism in Neurospora crassa

Mitogen-activated protein kinase ( MAPK) signaling cascades are composed of MAPK kinase kinases ( MAPKKKs), MAPK kinases ( MAPKKs), and MAPKs. In this study, we characterize components of a MAPK cascade in Neurospora crassa (mik-1, MAPKKK; mek-1, MAPKK; and mak-1, MAPK) homologous to that controlling cell wall integrity in Saccharomyces cerevisiae. Growth of basal hyphae is significantly reduced in mik-1, mek-1, and mak-1 deletion mutants on solid medium. All three mutants formed short aerial hyphae and the formation of asexual macroconidia was reduced in Delta mik-1 mutants and almost abolished in Delta mek-1 and Delta mak-1 strains. In contrast, the normally rare asexual spores, arthroconidia, were abundant in cultures of the three mutants. Delta mik-1, Delta mek-1, and Delta mak-1 mutants were unable to form protoperithecia or perithecia when used as females in a sexual cross. The MAK-1 MAPK was not phosphorylated in Delta mik-1 and Delta mek-1 mutants, consistent with the involvement of MIK-1, MEK-1, and MAK-1 in the same signaling cascade. Interestingly, we observed increased levels of mRNA and protein for tyrosinase in the mutants under nitrogen starvation, a condition favoring sexual differentiation. Tyrosinase is an enzyme that catalyzes production of the secondary metabolite L-DOPA melanin. These results implicate the MAK-1 pathway in regulation of development and secondary metabolism in filamentous fungi.