Three aspartic acid residues of polygalacturonase-inhibiting protein (PGIP) from Phaseolus vulgaris are critical for inhibition of Fusarium phyllophilum PG.

Polygalacturonase-inhibiting proteins (PGIPs) are plant cell wall proteins that specifically inhibit the activity of endopolygalacturonases (PGs) produced by fungi during the infection process. The interaction with PGIPs limits the destructive potential of PGs and may trigger plant defence responses through the release of elicitor active oligogalacturonides. In order to pinpoint the residues of PvPGIP2 from Phaseolus vulgaris involved in the interaction with PGs, we used site-directed mutagenesis to mutate the residues D131, D157 and D203, and tested for the inhibitory activity of the mutant proteins expressed in Pichia pastoris against Fusarium phyllophilum and Aspergillus niger PGs. Here, we report that mutation of these residues affects the inhibition capacity of PvPGIP2 against F. phyllophilum PG.

Structural requirements of endopolygalacturonase for the interaction with PGIP (polygalacturonase-inhibiting protein).

To invade a plant tissue, phytopathogenic fungi produce several cell wall-degrading enzymes; among them, endopolygalacturonase (PG) catalyzes the fragmentation and solubilization of homogalacturonan. Polygalacturonase-inhibiting proteins (PGIPs), found in the cell wall of many plants, counteract fungal PGs by forming specific complexes with them. We report the crystal structure at 1.73 A resolution of PG from the phytopathogenic fungus Fusarium moniliforme (FmPG). The structure of FmPG was useful to study the mode of interaction of the enzyme with PGIP-2 from Phaseolus vulgaris. Several amino acids of FmPG were mutated, and their contribution to the formation of the complex with PGIP-2 was investigated by surface plasmon resonance. The residues Lys-269 and Arg-267, located inside the active site cleft, and His-188, at the edge of the active site cleft, are critical for the formation of the complex, which is consistent with the observed competitive inhibition of the enzyme played by PGIP-2. The replacement of His-188 with a proline or the insertion of a tryptophan after position 270, variations that both occur in plant PGs, interferes with the formation of the complex. We suggest that these variations are important structural requirements of plant PGs to prevent PGIP binding.

Study of the mode of action of endopolygalacturonase from Fusarium moniliforme.

One endopolygalacturonase from Fusarium moniliforme was purified from the culture broth of a transformed strain of Saccharomyces cerevisiae. Its kinetic parameters and mode of action were studied on galacturonic acid oligomers and homogalacturonan. The dimer was not a substrate for the enzyme. The enzyme was shown to follow Michaelis-Menten behaviour towards the other substrates tested. Affinity and maximum rate of hydrolysis increased with increasing chain length, up to the hexamer or heptamer, for which V(max) was in the same range as with homogalacturonan. The enzyme was demonstrated to have a multi-chain attack mode of action and its active site included five subsites ranging from -3 to +2. The final products of hydrolysis of homogalacturonan were the monomer and the dimer of galacturonic acid.

Mutational analysis of beta-glucanase genes from the plant-pathogenic fungus Cochliobolus carbonum.

Two new beta-glucanase-encoding genes, EXG2 and MLG2, were isolated from the plant-pathogenic fungus Cochliobolus carbonum using polymerase chain reaction based on amino acid sequences from the purified proteins. EXG2 encodes a 46.6-kDa exo-beta1,3-glucanase and is located on the same 3.5-Mb chromosome that contains the genes of HC-toxin biosynthesis. MLG2 encodes a 26.8-kDa mixed-linked (beta1,3-beta1,4) glucanase with low activity against beta1,4-glucan and no activity against beta1,3-glucan. Specific mutants of EXG2 and MLG2 were constructed by targeted gene replacement. Strains with multiple mutations (genotypes exg1/mlg1, exg2/mlg1, mlg1/mlg2, and exg1/exg2/mlg1/mlg2) were also constructed by sequential disruption and by crossing. Total mixed-linked glucanase activity in culture filtrates of mlg1/mlg2 and exg1/exg2/mlg1/mlg2 mutants was reduced by approximately 73%. Total beta1,3-glucanase activity was reduced by 10, 54, and 96% in exg2, mlg1, and exg1/exg2/mlg1/mlg2 mutants, respectively. The quadruple mutant showed only a modest decrease in growth on beta1,3-glucan or mixed-linked glucan. None of the mutants showed any decrease in virulence.

Crystallization and preliminary X-ray diffraction study of the endo-polygalacturonase from Fusarium moniliforme.

Endo-polygalacturonases catalyze the fragmentation and solubilization of the homogalacturonan of the plant cell wall. These enzymes are extracellularly targeted glycoproteins produced by a number of organisms such as fungi, bacteria and plants, and are involved in both pathological and physiological processes. Single crystals of the endo-polygalacturonase from the phytopathogenic fungus Fusarium moniliforme were obtained by the vapour-diffusion method at 294 K. The starting material as well as the crystal consist of three forms with different degrees of glycosylation. The crystals belong to the orthorhombic space group P212121 and diffract to 1.9 A resolution on a synchrotron-radiation source under cryocooling conditions.

Mutagenesis of endopolygalacturonase from Fusarium moniliforme: histidine residue 234 is critical for enzymatic and macerating activities and not for binding to polygalacturonase-inhibiting protein (PGIP).

The sequence encoding the endopolygalacturonase (PG) of Fusarium moniliforme was cloned into the E. coli/yeast shuttle vector Yepsec1 for secretion in yeast. The recombinant plasmid (pCC6) was used to transform Saccharomyces cerevisiae strain S150-2B; transformed yeast cells were able to secrete PG activity into the culture medium. The enzyme (wtY-PG) was purified, characterized, and shown to possess biochemical properties similar to those of the PG purified from F. moniliforme. The wtY-PG was able to macerate potato medullary tissue disks and was inhibited by the polygalacturonase-inhibiting protein (PGIP) purified from Phaseolus vulgaris. The sequence encoding PG in pCC6 was subjected to site-directed mutagenesis. Three residues in a region highly conserved in all the sequences known to encode PGs were separately mutated: His 234 was mutated into Lys (H 234-->K), and Ser 237 and Ser 240 into Gly (S 237-->G and S 240-->G). Each of the mutated sequences was used to transform S. cerevisiae and the mutated enzymes were purified and characterized. Replacement of His 234 with Lys abolished the enzymatic activity, confirming the biochemical evidence that a His residue is critical for enzyme activity. Replacement of either Ser 237 or Ser 240 with Gly reduced the enzymatic activity to 48% and 6%, respectively, of the wtY-PG. When applied to potato medullary tissue, F. moniliforme PG and wtY-PG caused comparable maceration, while the variant PGs exhibited a limited (S 234-->G and S 240-->G) or null (H 234-->K) macerating activity. The interaction between the variant enzymes and the P. vulgaris PGIP was investigated using a biosensor based on surface plasmon resonance (BIAlite). The three variant enzymes were still able to interact and bind to PGIP with association constants comparable to that of the wild type enzyme.

Cytological localization of thePGIP genes in the embryo suspensor cells ofPhaseolus vulgavis L.

Polygalacturonase-inhibiting protein (PGIP) is a cell wall protein which inhibits fungalendopolygalacturonases. A small gene family encodesPGIP in the genome of common bean, as indicated by Southernblot experiments performed at high-stringency conditions. Southern-blot analysis of DNA extracted from different cultivars ofPhaseolus vulgaris and fromPhaseolus coccineus showed length polymorphism of the hybridizing restriction fragments. The cytological localization of thePGIP genes was determined in polytene chromosomes of theP. vulgaris embryo suspensor cells. In-situ hybridization experiments using the clonedPGIP gene revealed labelling over a single region of the pericentromeric heterochromatin of chromosome pair X, next to the euchromatin, suggesting thatPGIP gene family may be clustered in one chromosomal region.