The characterisation of nascent pectin complexes in pea plants
The structure and properties of a nascent pectin-xyloglucan complex in etiolated pea epicotyls were investigated. Membrane pellets were prepared and incubated with UDP-[14C] galactose, and were extracted with six different reagents: Tris-HC1 buffer (pH 7.4); 50mM EDTA/50mM PO4 (pH6.8) at 100°C; 50mM EDTA/59mM PO4 (pH6.8) at 25°C; phospholipase C with 100 ml of 0.1M Tris-HC1 pH7.4 at 25°C; trypsin at 25°C and 0.1% Triton X-100 at 25°C. The best extractant used to solubilise the pectin-complex from the pellets was 50nM EDTA/50mM PO4 pH 6.8 at 100°C. Aqueous solutions of pectic polyuronides tend to associate covalently into multichain aggregates. Because of the tendency of pectins to aggregate in solution, the effect of a number of eluents on the behaviour of the complex on gel filtration was studied. 10mM EDTA/10nM PO4/1M NaC1 (pH6.8) was chosen as the best eluent to minimise this aggregate formation. On gel filtration using Sepharose CL-2B with this eluent, the complex eluted with a Kav of around 0.8, corresponding to a molecular size approximately 200 kDa, as judged by dextran standards. In other solvents tested, aggregation appeared to occur. On Sepharose CL-4B and CL-6B columns using the same EDTA/Pi/NaC1 solvent, the apparent molecular size was significantly reduced by a xyloglucan-specific endoglucanase, confirming the presence of a xyloglucan rather that a glucan in the complex. Polygalacturonase caused a greater decrease in apparent molecular size, to 10-20 kDa while endo-1 4-b-galactanase converted the radioactivity to [14C] galactobiose and [14C] galactose. When the [14C] galactose-labelled complex was incubated in solution with 3MM paper, the radioactivity was almost completely absorbed onto the paper over a period of about 6 hours. This confirmed the presence of xyloglucan attached to pectin. A new model for the structure of the nascent pectin-xyloglucan complex is proposed.