Recent efforts at the proteomic level were employed to describe the protein equipment of the plasma membrane of the model plant Arabidopsis thaliana. These studies had revealed that the plasma membrane is rich in extrinsic proteins but came up against two major problems: (i) few hydrophobic proteins were recovered in two-dimensional electrophoresis gels, and (ii) many plasma membrane proteins had no known function or were unknown in the database despite extensive sequencing of the Arabidopsis genome. In this paper, several methods expected to enrich a membrane sample in hydrophobic proteins were compared. The optimization of solubilization procedures revealed that the detergent to be used depends on the lipid content of the sample. The corresponding proteomes were compared with the statistical model AMMI (additive main effects with multiplicative interaction) that aimed at regrouping proteins according to their solubility and electrophoretic properties. Distinct groups emerged from this analysis and the identification of proteins in each group allowed us to assign specific features to several of them. For instance, two of these groups regrouped very hydrophobic proteins, one group contained V-ATPase subunits, another group contained proteins with one transmembrane domain as well as proteins known to interact with membrane proteins. This study provides methodological tools to study particular classes of plasma membrane proteins and should be applicable to other cellular membranes.

[1] Results from recent field studies in Antarctic sea ice show no clear differences in dissolved iron (dFe) concentrations between pack ice sampled in East Antarctica (2.6-20.5 nmol/L), in the Weddell Sea (0.7-36.8 nmol/L), and in the Bellingshausen Sea (1.1-30.2 nmol/L). Dissolved Fe concentrations were also similar in land-fast ice collected in East Antarctica (0.7-4.3 nmol/L) and in the Ross Sea (1.1-6.0 nmol/L). In contrast, we observed a remarkable seasonal drawdown of dFe in sea ice for all reported studies. Furthermore, large inter-annual variations in depth-averaged dFe and organic matter concentrations were observed in sea ice collected in the East Antarctic sector between expeditions in late austral winter-spring of 2003 and 2007. Variability in the water column productivity and in the magnitude of the "new" Fe supply (e.g., upwelling, resuspended shelf sediments) at the time of sea ice formation could explain such differences.

No abstract