title: Functional phosphoproteomic profiling of phosphorylation sites in membrane fractions of salt-stressed Arabidopsis thaliana.
abstract: BACKGROUND
  Under conditions of salt stress, plants respond by initiating phosphorylation cascades. Many key phosphorylation events occur at the membrane. However, to date only limited sites have been identified that are phosphorylated in response to salt stress in plants.
  
  
  RESULTS
  Membrane fractions from three-day and 200 mM salt-treated Arabidopsis suspension plants were isolated, followed by protease shaving and enrichment using Zirconium ion-charged magnetic beads, and tandem mass spectrometry analyses. From this isolation, 18 phosphorylation sites from 15 Arabidopsis proteins were identified. A unique phosphorylation site in 14-3-3-interacting protein AHA1 was predominately identified in 200 mM salt-treated plants. We also identified some phosphorylation sites in aquaporins. A doubly phosphorylated peptide of PIP2;1 as well as a phosphopeptide containing a single phosphorylation site (Ser-283) and a phosphopeptide containing another site (Ser-286) of aquaporin PIP2;4 were identified respectively. These two sites appeared to be novel of which were not reported before. In addition, quantitative analyses of protein phosphorylation with either label-free or stable-isotope labeling were also employed in this study. The results indicated that level of phosphopeptides on five membrane proteins such as AHA1, STP1, Patellin-2, probable inactive receptor kinase (At3g02880), and probable purine permease 18 showed at least two-fold increase in comparison to control in response to 200 mM salt-stress.
  
  
  CONCLUSIONS
  In this study, we successfully identified novel salt stress-responsive protein phosphorylation sites from membrane isolates of abiotic-stressed plants by membrane shaving followed by Zr4+-IMAC enrichment. The identified phosphorylation sites can be important in the salt stress response in plants.
authors.affiliation: Graduate Institute of Biotechnology, National Pingtung University of Science and Technology, Taiwan. jlhsu@mail.npust.edu.tw
authors.forename: Jue-Liang
authors.initials: JL
authors.lastname: Hsu
authors.forename: Lan-Yu
authors.initials: LY
authors.lastname: Wang
authors.forename: Shu-Ying
authors.initials: SY
authors.lastname: Wang
authors.forename: Ching-Huang
authors.initials: CH
authors.lastname: Lin
authors.forename: Kuo-Chieh
authors.initials: KC
authors.lastname: Ho
authors.forename: Fong-Ku
authors.initials: FK
authors.lastname: Shi
authors.forename: Ing-Feng
authors.initials: IF
authors.lastname: Chang