Association of N- and C-terminal domains of phospholipase D is required for catalytic activity

Aluminum fluoride inhibition of cabbage phospholipase D by a phosphate-mimicking mechanism

Aluminum fluoride (AlF(4)(-)) inhibited phospholipase D (PLD) purified from cabbage in both PIP(2)-dependent and PIP(2)-independent assays, consistent with its previously observed effect on mammalian PLD. The possibility that AlF(4)(-) may exert this effect through its known phosphate-mimicking property was examined. Inorganic phosphate, as well as two phosphate analogs, beryllium fluoride and orthovanadate, also inhibited cabbage PLD. Enzyme kinetic studies confirmed that PLD followed Hill kinetics, characteristic for allosteric enzymes, with an apparent Hill coefficient (n(app)) of 3.8, indicating positive cooperativity among multiple substrate-binding sites and suggesting possible functional oligomerization of the enzyme. AlF(4)(-) modification of PLD kinetics was consistent with a competitive mode of enzyme inhibition. It is therefore proposed that AlF(4)(-), and other phosphate analogs, inhibits plant PLD by competing with a substrate phosphate group for a substrate-binding site, thereby preventing the formation of an enzyme-phosphatidyl intermediate. This may be a conserved feature of PLD superfamily enzymes.

Midportion antibodies stimulate glycosylphosphatidylinositol-specific phospholipase D activity

Limited information is known regarding the regulation, structural features, and functional domains of glycosylphosphatidylinositol-specific phospholipase D (GPI-PLD, EC 3. 1.4.50). Previous studies demonstrated that trypsin cleavage of GPI-PLD at or near Arg325 and/or Arg589 in bovine serum GPI-PLD was associated with an increase in enzymatic activity. Since the Arg325 is predicted to be in a region between the catalytic domain and predicted beta-propeller structure in the C-terminal portion of GPI-PLD (T. A. Springer, Proc. Natl. Acad. Sci. USA 94, 65-72, 1997), we hypothesized that this connecting region is important for catalytic activity. Trypsin cleavage of human serum GPI-PLD, which has an Arg325 but lacks the Arg589 present in bovine serum GPI-PLD, also increased GPI-PLD activity. Peptide-specific antibodies to residues 275-296 (anti-GPI-PLD(275)) and a monoclonal antibody, 191, with an epitope encompassing Arg325, also stimulated GPI-PLD activity. Pretreating human GPI-PLD with trypsin demonstrated that anti-GPI-PLD(275) only stimulated the activity of intact GPI-PLD. These results suggest that trypsin activation and anti-GPI-PPLD(275) may have similar effects on GPI-PLD. Consistent with this is the observation that both manipulations decreased the affinity of GPI-PLD for mixed micelle substrates. These results indicate that the midportion region of GPI-PLD is important in regulating enzymatic activity.

Location of the catalytic nucleophile of phospholipase D of Streptomyces antibioticus in the C-terminal half domain

Phospholipase D (PLD) of Streptomyces antibioticus was labelled with fluorescent-labelled substrate, 1-hexanoyl-2-{6-[(7-nitro-2-1, 3-benzoxadiazol-4-yl)-amino]hexanoyl}-sn-glycero-3-phosphocholine, when it was incubated with the substrate and the reaction followed by SDS/PAGE. Mutant enzymes lacking the catalytic activity were not labelled under the same conditions, indicating that labelling of the PLD occurred as the result of its catalytic action. This confirmed that the labelled protein was the phosphatidyl PLD intermediate. PLDs contain two copies of the highly conserved catalytic HxKxxxxD (HKD) motif. Therefore, two protein fragments were separately prepared with recombinant strains of Escherichia coli. One of the fragments was the N-terminal half of the intact PLD containing one HKD motif, and the other was the C-terminal half with the other motif. An active enzyme was reconstructed from these two fragments, and therefore designated fragmentary PLD (fPLD). When fPLD was subjected to the labelling experiment, only the C-terminal half was labelled. Therefore, it was concluded that the catalytic nucleophile that bound directly to the phosphatidyl group of the substrate was located on the C-terminal half of PLD, and that the N-terminal half did not contain such a nucleophile.

The PLD superfamily: insights into catalysis

Knowledge of the PLD superfamily is rapidly expanding and new insights into the mechanism and regulation of the superfamily are rapidly emerging. The recent structural analysis and use of mutant proteins suggest a mechanism that involves two active sites acting in concert. While a number of residues are required for activity, it appears most likely that a histidine is the residue that becomes covalently linked to phosphatidate in catalysis. Evidence for these proposals is covered in this article.

Expression, characterization, and mutagenesis of the Yersinia pestis murine toxin, a phospholipase D superfamily member

A phospholipase D (PLD) superfamily was recently identified that contains proteins of highly diverse functions with the conserved motif HXKX4DX6G(G/S). The superfamily includes a bacterial nuclease, human and plant PLD enzymes, cardiolipin synthases, phosphatidylserine synthases, and the murine toxin from Yersinia pestis (Ymt). Ymt is particularly effective as a prototype for family members containing two conserved motifs, because it is smaller than many other two-domain superfamily enzymes, and it can be overexpressed. Large quantities of pure recombinant Ymt allowed the formation of diffraction-quality crystals for x-ray structure determination. Dimeric Ymt was shown to have PLD-like activity as demonstrated by the hydrolysis of phosphatidylcholine. Ymt also used bis(para-nitrophenol) phosphate as a substrate. Using these substrates, the amino acids essential for Ymt function were determined. Specifically, substitution of histidine or lysine in the conserved motifs reduced the turnover rate of bis(para-nitrophenol) phosphate by a factor of 10(4) and phospholipid turnover to an undetectable level. The role of the conserved residues in catalysis was further defined by the isolation of a radiolabeled phosphoenzyme intermediate, which identified a conserved histidine residue as the nucleophile in the catalytic reaction. Based on these data, a unifying two-step catalytic mechanism is proposed for this diverse family of enzymes.