Poster: Membrane Transport

Abs # 1219: Plasma membrane association of proteins of the SOS signaling pathway for Na+ homeostasis.

Presenter:	Pardo, Jose M., pardo@cica.es
Authors	Pardo, Jose M. (A)   Quintero, Francisco J. (A)
Affiliations:	(A): Inst. Recursos Naturales y Agrobiologia, Consejo Superior de Investigaciones Cientificas, Spain

The Arabidopsis thaliana SOS1 protein is a plasma membrane Na+/H+ antiporter that functions in the extrusion and long distance transport of sodium. sos1 mutant plants are salt-sensitive and deficient in potassium nutrition at low external concentrations. These phenotypic traits are shared with mutants lacking the protein kinase SOS2 and the Ca2+-binding protein SOS3. We have reconstituted the SOS system in yeast cells to show that the three SOS proteins function in the same response pathway. Co-expression of SOS2 and SOS3 dramatically increased the Na+ tolerance of yeast cells in a SOS1-dependent manner, whereas SOS2 or SOS3 had no effect individually. The SOS2/SOS3 kinase complex phosphorylated the SOS1 protein and stimulated its transport activity. In addition to activating the SOS2 kinase, SOS3 functions to recruits SOS2 to the plasma membrane, and this additional role requires post-translational modifications of the SOS3 protein. Myristoylation of SOS3 is essential for plasma membrane targeting of the SOS2 kinase but not for the formation of the SOS2/SOS3 complex. On the other hand, palmitoylation of SOS3 was dispensable but contributed to the full association of SOS2 with the plasma membrane. The fact that palmitoylation is a reversible modification may provide a mechanism for regulation of the subcellular localization and/or interaction between SOS3 and SOS2. Our results are the first example of functional reconstitution of a plant response pathway in a heterologous system and demonstrate that the ion transporter SOS1, the protein kinase SOS2 and its associated Ca2+ sensor SOS3, constitute a functional module associated with the plasma membrane for the regulation of sodium transport.