Submitted to: Molecular Plant-Microbe Interactions
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/20/2007
Publication Date: 1/20/2008
Citation: Klosterman, S.J., Martinez-Espinoza, A.D., Andrews, D.L., Seay, J.R., Gold, S.E. 2008. Ubc2, an Ortholog of the Yeast Ste50p Adaptor, Possesses a Basidiomycete-Specific Carboxy terminal Extension Essential for Pathogenicity Independent of Pheromone Response. Molecular Plant-Microbe Interactions. Vol. 21, No. 1 p. 110-121. Interpretive Summary: The fungus Ustilago maydis is a model system for the study of plant-pathogen interactions and is related to other economically important fungi such as the mushrooms and rusts collectively known as basidiomycetes. Ustilago maydis alternates between a saprophytic phase and a biotrophic phase in which it depends on living tissue for its development. Proteins involved in controlling mating, morphogenesis and pathogenicity have been identified previously in the fungus Ustilago maydis. In this study, one of these Ustilago maydis proteins known as Ubc2 was investigated at the level of its individual domains for its role in morphogenesis, mating and pathogenicity. The results of this study indicate that two of these domains at the amino terminus of this adaptor protein are required for filamentous growth while the two Src homology 3 domains at the carboxy terminus are required for pathogenicity independent of the mating response pathway. Because related proteins from other fungi lack the two carboxy terminal domains, signaling via these domains in this group of proteins is a characteristic specific to the basidiomycete fungi.
Technical Abstract: Proteins involved in the 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 SAM and 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 MAPKK 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 S. cereviseae. In contrast, C-terminal deletion mutants were fully capable of filamentous growth and mating. However, surprisingly these strains were non-pathogenic. 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 have additionally integrated C-terminal SH3 domains that are critical for additional signal transduction mechanisms including those that lead to pathogenesis.