Ubc2, an ortholog of the yeast Ste50p adaptor, possesses a basidiomycete-specific carboxy terminal extension essential for pathogenicity independent of pheromone response.
Proteins involved in the mitogen-activated protein (MAP) kinase pathway controlling mating, morphogenesis, and pathogenicity have been identified previously in the fungus Ustilago maydis. One of these, the Ubc2 adaptor protein, possesses a basidiomycete-specific structure. In addition to containing sterile alpha motif (SAM) and ras association (RA) domains typical of Ste50-like adaptor proteins found in the fungal phylum Ascomycota, Ubc2 also contains two C-terminal SH3 domains. Yeast two-hybrid assays indicated that Ubc2 interacts with the MAP kinase-kinase kinase Ubc4 via the SAM domains at each of their respective N-termini. Site-directed mutagenesis of ubc2 and complementation analyses revealed that the SAM and RA domains of Ubc2 are essential for filamentous growth. These data support a role for the ascomycete-like N-terminus of Ubc2 in regulating pheromone-responsive mating and morphogenesis analogous to the role of Ste50p in Saccharomyces cerevisiae. In contrast, C-terminal deletion mutants were fully capable of filamentous growth and mating. However, surprisingly, these strains were nonpathogenic. Further, directed mutagenesis of the C-terminus revealed that both SH3 domains are required for pathogenicity. These results suggest that the Basidiomycota have retained the mating and morphogenetic functions of Ste50-type proteins in the N-terminal half of their Ubc2-type adaptors but, additionally, have integrated C-terminal SH3 domains that are critical for additional signal transduction mechanisms, including those that lead to pathogenesis.
Klosterman
Steven J
SJ
Department of Plant Pathology, University of Georgia, Athens, GA 30602, USA.
Martinez-Espinoza
Alfredo D
AD
Andrews
David L
DL
Seay
Jeffrey R
JR
Gold
Scott E
SE
D048868:Adaptor Proteins, Signal Transducing / Q000737:chemistry / Q000378:metabolism; D017124:Conserved Sequence; D005656:Fungal Proteins / Q000737:chemistry* / Q000378:metabolism*; D049770:Genes, Mating Type, Fungal; D005816:Genetic Complementation Test; D020930:MAP Kinase Kinase Kinases / Q000378:metabolism; D020928:Mitogen-Activated Protein Kinases / Q000737:chemistry* / Q000378:metabolism*; D008954:Models, Biological; D016297:Mutagenesis, Site-Directed; D010675:Pheromones / Q000378:metabolism*; D010802:Phylogeny; D011485:Protein Binding; D012441:Saccharomyces cerevisiae / Q000737:chemistry*; D029701:Saccharomyces cerevisiae Proteins / Q000737:chemistry*; D017386:Sequence Homology, Amino Acid; D013045:Species Specificity; D020798:Two-Hybrid System Techniques; D014588:Ustilago / Q000166:cytology / Q000201:enzymology / Q000472:pathogenicity*; D018909:src Homology Domains
D048868:Adaptor Proteins, Signal Transducing; D005656:Fungal Proteins; D010675:Pheromones; C079506:STE50 protein, S cerevisiae; D029701:Saccharomyces cerevisiae Proteins; C441899:ubc2 protein, Ustilago maydis; D020928:Mitogen-Activated Protein Kinases; D020930:MAP Kinase Kinase Kinases