1Department of Biochemistry and Biophysics, Texas A&M University, College Station 77843-2128, U.S.A.; 2Department of Horticulture, Texas A&M University; 3Universidad Pablo de Olavide, Carretera de Utrera Km 1 Sevilla, 41013 Spain; 4Instituto de Recursos Naturales Y Agrobiologia de Sevilla, Campus de Reina Mercedes, Apartado 1052, Sevilla, 41080 Spain; 5Center for Plant Environmental Stress Physiology, 1165 Horticulture Building, Purdue University, West Lafayette, IN 47907-1165, U.S.A.
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Accepted 17 February 2004.
Antimicrobial activities of many defense proteins are profoundly altered by inorganic cations, thereby controlling disease pathologies in a number of mammalian systems, such as cystic fibrosis in humans. Protein-based active defense systems in plants also are influenced by cations; however, little is known of how these cation effects are mediated. Cytotoxicity of the pathogenesis-related protein osmotin against the model fungus Saccharomyces cerevisiae was progressively abolished by K+. By the use of S. cerevisiae mannosylation mutants, this effect was shown to require mannosephosphate residues in the cell wall. However, osmotin activity was not suppressed by even high concentrations of Ca2+. Rather, submillimolar levels of Ca2+ specifically facilitated osmotin's activity, as well as its binding to the cell surface. This effect also was dependent on mannosephosphate groups on the cell surface, and appeared to require negative charge on a portion of the osmotin protein. Results suggest that Ca2+ modulates osmotin action by facilitating its binding to the fungal cell surface, but that K+ blocks this interaction by competing for binding to mannosephosphate groups. Therefore, we have identified glycan interaction as a mechanism for antimicrobial protein activity modulation by cations, a pattern that may apply to diverse innate defense responses.
© 2004 The American Phytopathological Society