6 pages, 6 figures, 29 references.
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.
This work was
supported by Spanish Ministerio de Ciencia y Tecnología Grant
BIO2000-0938 (to F.J.Q. and J.M.P.) and by National Institutes of
Health Grant R01GM59138 (to J.-K.Z.).
Peer reviewed