Plant plasma membrane 14-3-3 proteins differ in solubility and form fusicoccin-dependent complexes

The binding protein for the phytotoxin fusicoccin belongs to the class of highly conserved 14-3-3 proteins. A general principle for the mode of action of 14-3-3 proteins is that they serve as docking clamps in order to facilitate protein interactions. This implies that 14-3-3 proteins may behave according to the proteins they interact with. Plasma membrane (PM) 14-3-3 proteins from oat roots indeed exhibited different properties, for example with respect to detergent solubility. The most abundant 14-3-3ʹs with apparent molecular masses of 30 and 31 kDa, could be largely solubilized from the plasma membrane in high detergent buffer. However, part of these solubilized 14-3-3ʹs precipitated when the detergent concentration was reduced. Moreover, a minor 35 kDa 14-3-3 protein was soluble in high detergent buffer but also completely precipitated at low detergent buffer. We present evidence that this 35 kDa 14-3-3 protein is phosphorylated and that the lower mobility in SDS-PAGE is likely to be due to binding of Ca2+ by the phosphorylated form of the protein. After solubilization in high detergent, the 14-3-3 proteins are present both as dimers and as part of a large protein complex, as shown by size exclusion chromatography. Treatment of PM with fusicoccin prior to solubilization resulted in a shift of the amount of 14-3-3 present in dimeric form towards the large molecular size complex. The consequences of the properties of plasma membrane 14-3-3 proteins described above is discussed with respect to 14-3-3 proteins present in the cytoplasm and nucleus.