Abstract
BackgroundThe PolyGalacturonase-Inhibiting Proteins (PGIP) of plant cell wall limit the invasion of phytopathogenic organisms by interacting with the enzyme PolyGalacturonase (PG) they secrete to degrade pectin present in the cell walls. PGIPs from different or same plant differ in their inhibitory activity towards the same PG. PGIP2 from Phaseolus vulgaris (Pv) inhibits the PG from Fusarium moniliforme (Fm) although PGIP1, another member of the multigene family from the same plant sharing 99% sequence similarity, cannot. Interestingly, PGIP3 from Glycine max (Gm) which is a homologue of PGIP2 is capable of inhibiting the same PG although the extent of similarity is lower and is 88%. It therefore appears that subtle changes in the sequence of plant PGIPs give rise to different specificity for inhibiting pathogenic PGs and there exists no direct dependence of function on the extent of sequence similarity.ResultsStructural information for any PGIP-PG complex being absent, we resorted to molecular modelling to gain insight into the mechanism of recognition and discrimination of PGs by PGIPs. We have built homology models of PvPGIP1 and GmPGIP3 using the crystal structure of PvPGIP2 (1OGQ) as template. These PGIPs were then docked individually to FmPG to elucidate the characteristics of their interactions. The mode of binding for PvPGIP1 to FmPG considerably differs from the mode observed for PvPGIP2-FmPG complex, regardless of the high sequence similarity the two PGIPs share. Both PvPGIP2 and GmPGIP3 despite being relatively less similar, interact with residues of FmPG that are known from mutational studies to constitute the active site of the enzyme. PvPGIP1 tends to interact with residues not located at the active site of FmPG. Looking into the electrostatic potential surface for individual PGIPs, it was evident that a portion of the interacting surface for PvPGIP1 differs from the corresponding region of PvPGIP2 or GmPGIP3.Conclusionvan der Waals and eletrostatic interactions play an active role in PGIPs for proper recognition and discrimination of PGs. Docking studies reveal that PvPGIP2 and GmPGIP3 interact with the residues constituting the active site of FmPG with implications that the proteins bind/block FmPG at its active site and thereby inhibit the enzyme.
Highlights
The PolyGalacturonase-Inhibiting Proteins (PGIP) of plant cell wall limit the invasion of phytopathogenic organisms by interacting with the enzyme PolyGalacturonase (PG) they secrete to degrade pectin present in the cell walls
Four out of the eight variations between PvPGIP1 and PvPGIP2 fall in the repeat regions and the rest reside in the nonLRR region of
In order to obtain experimentally relevant information concerning the thermodynamic properties of binding, we wish to work in future, with advanced simulation methods such as free energy perturbation or thermodynamic integration to assess the binding free energy changes upon mutation of the important residues in PGIP. It has been noted from our study on sequence analysis that 50% of the sequence variation between PvPGIP1 and PvPGIP2 fall in the non-Leucine-rich Repeat (LRR) region whereas only 15% of the variation in GmPGIP3 with PvPGIP2 fall in the non-LRR region
Summary
The PolyGalacturonase-Inhibiting Proteins (PGIP) of plant cell wall limit the invasion of phytopathogenic organisms by interacting with the enzyme PolyGalacturonase (PG) they secrete to degrade pectin present in the cell walls. PolyGalacturonase-Inhibiting Protein (PGIP) is one among the pathogenesis-related (PR) proteins that is found at the cell surface of plant cells. It binds and inhibits the enzyme PolyGalacturonase (PG) from the invading pathogen which could be fungus, insect or bacterium, preventing its colonization in the host cell and the progress of the disease [1,2]. PolyGalacturonases (PG) are a class of pectinolytic enzyme secreted by the pathogen at the early stages of infection to depolymerize homogalacturonan (HGA), the main component of pectin in the plant cell wall [3]. HGA is the 1, 4 linked alpha-D-galactosyluronic acid polymer found in the plant cell which forms the first line of barrier and thereby plays a critical role in controlling pathogen invasion [4,5,6,7]
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