title: Reconstitution in yeast of the Arabidopsis SOS signaling pathway for Na+ homeostasis.
abstract: The Arabidopsis thaliana SOS1 protein is a putative Na+/H+ antiporter that functions in Na+ extrusion and is essential for the NaCl tolerance of plants. sos1 mutant plants share phenotypic similarities with mutants lacking the protein kinase SOS2 and the Ca2+ sensor SOS3. To investigate whether the three SOS proteins function in the same response pathway, we have reconstituted the SOS system in yeast cells. Expression of SOS1 improved the Na+ tolerance of yeast mutants lacking endogenous Na+ transporters. Coexpression of SOS2 and SOS3 dramatically increased SOS1-dependent Na+ tolerance, whereas SOS2 or SOS3 individually had no effect. The SOS2/SOS3 kinase complex promoted the phosphorylation of SOS1. A constitutively active form of SOS2 phosphorylated SOS1 in vitro independently of SOS3, but could not fully substitute for the SOS2/SOS3 kinase complex for activation of SOS1 in vivo. Further, we show that SOS3 recruits SOS2 to the plasma membrane. Although sos1 mutant plants display defective K+ uptake at low external concentrations, neither the unmodified nor the SOS2/SOS3-activated SOS1 protein showed K+ transport capacity in vivo, suggesting that the role of SOS1 on K+ uptake is indirect. Our results provide an example of functional reconstitution of a plant response pathway in a heterologous system and demonstrate that the SOS1 ion transporter, the SOS2 protein kinase, and its associated Ca2+ sensor SOS3 constitute a functional module. We propose a model in which SOS3 activates and directs SOS2 to the plasma membrane for the stimulatory phosphorylation of the Na+ transporter SOS1.
authors.affiliation: Instituto de Recursos Naturales y Agrobiologia, Consejo Superior de Investigaciones Cientificas, Sevilla 41080, Spain.
authors.forename: Francisco J
authors.initials: FJ
authors.lastname: Quintero
authors.forename: Masaru
authors.initials: M
authors.lastname: Ohta
authors.forename: Huazhong
authors.initials: H
authors.lastname: Shi
authors.forename: Jian-Kang
authors.initials: JK
authors.lastname: Zhu
authors.forename: Jose M
authors.initials: JM
authors.lastname: Pardo
mesh_terms: D017360:Arabidopsis / Q000502:physiology*; D029681:Arabidopsis Proteins / Q000378:metabolism / Q000502:physiology*; D001483:Base Sequence; D002462:Cell Membrane / Q000378:metabolism; D017931:DNA Primers; D006706:Homeostasis; D010766:Phosphorylation; D015398:Signal Transduction; D012964:Sodium / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D017931:DNA Primers; D012964:Sodium