Isolation and Characterization From Pathogenic Fungi of Genes Encoding Ammonium Permeases and Their Roles in Dimorphism

Mol Microbiol. 2003 Oct;50(1):259-75. doi: 10.1046/j.1365-2958.2003.03680.x.

Abstract

Nutrient sensing plays important roles in fungal development in general, and specifically in critical aspects of pathogenicity and virulence, for both animal and plant pathogens. Dimorphic pathogens such as the phytopathogenic smut fungi, Ustilago maydis and Microbotryum violaceum, must switch from a yeast-like to a filamentous form in order to cause disease. Two genes encoding methylammonium permeases (MEPs) were identified from each of these latter fungi and all the encoded proteins were most similar to Mep2p, the high-affinity permease from Saccharomyces cerevisiae that plays a direct role in pseudohyphal or filamentous growth for that organism. This is the first report of MEPs from pathogenic fungi. The two genes from U. maydis and one of the genes from M. violaceum were expressed in diploid S. cerevisiae mutants deleted for all three mep genes (mep1mep2mep3). Each of the heterologous genes could complement the severe growth defect of the S. cerevisiae mutant on low ammonium. Moreover, the U. maydis ump2 gene, initially detected as an upregulated gene in budding cells, was also able to complement the pseudohyphal defect characteristic of the mutant yeast. This gene is thus one of few heterologous MEP genes capable of efficiently restoring pseudohyphal growth in yeast. For U. maydis, disruption of ump2 eliminated the filamentous phenotype of haploid cells on low ammonium, while ump1 disruption only slightly reduced methylamine uptake. The most significant drop in methylamine uptake was seen for the ump2 and the ump1ump2 double mutants. Moreover, when grown in liquid medium, the ump1ump2 double mutant aggregated and sedimented. Also, the importance of a putative site for phosphorylation by protein kinase A was investigated in both Mep2p and Ump2p via site-directed mutagenesis of the respective genes. A mutation predicted to prevent phosphorylation of either protein, still allowed each to provide growth on low ammonium, but eliminated their abilities to provide pseudohyphal growth for the S. cerevisiae triple mutant. These findings allow us to present a model of how ammonium transporters play a role in regulating dimorphic growth in fungi.

Publication types

  • Research Support, U.S. Gov't, Non-P.H.S.

MeSH terms

  • Amino Acid Sequence
  • Basidiomycota / cytology
  • Basidiomycota / genetics
  • Basidiomycota / growth & development*
  • Basidiomycota / metabolism
  • Carrier Proteins / genetics
  • Cation Transport Proteins / genetics
  • Cyclic AMP-Dependent Protein Kinases / metabolism
  • Fungal Proteins / genetics
  • Fungal Proteins / metabolism
  • Gene Deletion
  • Gene Expression Regulation, Fungal
  • Genes, Fungal
  • Genetic Complementation Test
  • Membrane Transport Proteins / genetics*
  • Membrane Transport Proteins / metabolism
  • Methylamines / metabolism*
  • Molecular Sequence Data
  • Mutation
  • Nitrogen / metabolism
  • Phosphorylation
  • Saccharomyces cerevisiae / genetics
  • Saccharomyces cerevisiae Proteins / genetics
  • Sequence Homology
  • Ustilago / cytology
  • Ustilago / genetics
  • Ustilago / growth & development*
  • Ustilago / metabolism

Substances

  • Carrier Proteins
  • Cation Transport Proteins
  • Fungal Proteins
  • MEP1 protein, S cerevisiae
  • MEP2 protein, S cerevisiae
  • MEP3 protein, S cerevisiae
  • Membrane Transport Proteins
  • Methylamines
  • Saccharomyces cerevisiae Proteins
  • methylamine
  • Cyclic AMP-Dependent Protein Kinases
  • Nitrogen