Purification and characterisation of cell wall acid phosphatases of roots of white clover (Trifolium repens L.) : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Biology at Massey University, Palmerston North, New Zealand
Plants of white clover (Trifolium repens L., cultivar Huia, genotype PgH2) were either grown in half-strength Hoaglands solution (P-containing media) or subjected to phosphate starvation by omitting the sole source phosphate (KH2PO4) from Hoaglands solution media for a period of five weeks. The phosphate content of the first fully expanded leaf was determined in plants from both treatments. After 2 weeks, the P content in leaves from plants grown in P-deprived media was significantly lower (p < 0.001) than the P-supplied plants, and continued to decrease over the 5-week time course. Ionically bound acid phosphatases were extracted with 1 M NaCl from the cell walls of roots. In roots of plants maintained in P-deprived media, acid phosphatase activity increased over the 5-week time course, while the activity in roots of plants grown in P-containing media did not change. After four weeks in P-deprived media, the cell wall ionically-bound acid phosphatase fraction was subjected to hydrophobic column chromatography and two distinct acid phosphatases (designated Apase I and Apase II) identified. There is a temporal difference in induction of Apase I and Apase II. After one week of P-deprivation, the activity of Apase II reached its maximum and did not increase further in following weeks. The activity of Apase I was only half that of Apase II after one week of P-deprivation, but increased continually to be significantly higher than the activity of Apase II by the end of week 4. Apase I and II were further purified using gel filtration column chromatography, and each enzyme subsequently separated further into two isoforms by ion-exchange chromatography. Both isoforms of Apase I (Ia and Ib) exist as active monomers of 52 kD as determined by SDS-PAGE and by gel filtration. For Apase II, both isoforms (IIa and IIb) also exist as active monomers of 112 kD as determined by SDS-PAGE and 92 kD by gel filtration. Both Ia and Ib are glycosylated as determined by recognition by a Galanthus nivalis (GNA) lectin (which recognises terminal mannose or oligomannose N-linked glycan chains) or by a monoclonal antibody YZ1/2.23 (which recognises xylose/fucose-containing complex-type glycan chains). Apase Ia was recognised by both sugar probes, while Apase Ib was recognised by YZ1/2.23 only. Apase IIa was not recognized by either of sugar probes, while Apase IIb is a glycoprotein as determined by recognition by YZ1/2.23. Using ρNPP as substrate, the pH optima for Apase Ia, Ib, IIa and IIb are 5.8, 6.2, 5.8 and 6.8, respectively. Isoelectric focusing determined that Apase Ia split into two bands with pI values of 7.0 and 7.3, Apase Ib showed a major band with a pI of 6.7, Apase IIa showed a single band with a pI of 4.4 and Apase IIb split into two closely located bands with pI values of 5.2 and 5.3. The activity of all four isoforms was severely inhibited by Cu2+, Zn2+ and molybdate. Fe3+ is also an inhibitor but not as potent as the other three metal ions. Co2+ and Al3+ displayed greater inhibition of Apase I when compared with Apase II. Tartrate and EDTA had no effect on the activity of all four isoforms, but inorganic phosphate is a strong inhibitor of all the four isoforms. Each of the four isoforms showed a broad range of substrate specificity, with ATP and PPi the preferred substrates, and PEP and 3-PGA the least preferred substrates. All four isoforms showed no hydrolysis activity toward phytic acid. A short sequence containing 5 amino acid residues was obtained from Apase Ib, but no significant sequence identity with any existing protein sequence was found.