Experimental and bioinformatic characterization of a recombinant polygalacturonase-inhibitor protein from pearl millet and its interaction with fungal polygalacturonases.

S Ashok Prabhu,Ratna Singh,Stephan Kolkenbrock,Ramachandra K Kini,Martin Wagenknecht,Neerakkal Sujeeth,Bruno M Moerschbacher,Nour Eddine El Gueddari

doi:10.1093/jxb/eru266

Abstract

Polygalacturonases (PGs) are hydrolytic enzymes employed by several phytopathogens to weaken the plant cell wall by degrading homopolygalacturonan, a major constituent of pectin. Plants fight back by employing polygalacturonase-inhibitor proteins (PGIPs). The present study compared the inhibition potential of pearl millet PGIP (Pennisetum glaucum; PglPGIP1) with the known inhibition of Phaseolus vulgaris PGIP (PvPGIP2) against two PGs, the PG-II isoform from Aspergillus niger (AnPGII) and the PG-III isoform from Fusarium moniliforme (FmPGIII). The key rationale was to elucidate the relationship between the extent of sequence similarity of the PGIPs and the corresponding PG inhibition potential. First, a pearl millet pgip gene (Pglpgip1) was isolated and phylogenetically placed among monocot PGIPs alongside foxtail millet (Setaria italica). Upstream sequence analysis of Pglpgip1 identified important cis-elements responsive to light, plant stress hormones, and anoxic stress. PglPGIP1, heterologously produced in Escherichia coli, partially inhibited AnPGII non-competitively with a pH optimum between 4.0 and 4.5, and showed no inhibition against FmPGIII. Docking analysis showed that the concave surface of PglPGIP1 interacted strongly with the N-terminal region of AnPGII away from the active site, whereas it weakly interacted with the C-terminus of FmPGIII. Interestingly, PglPGIP1 and PvPGIP2 employed similar motif regions with few identical amino acids for interaction with AnPGII at non-substrate-binding sites; however, they engaged different regions of AnPGII. Computational mutagenesis predicted D126 (PglPGIP1)-K39 (AnPGII) to be the most significant binding contact in the PglPGIP1-AnPGII complex. Such protein-protein interaction studies are crucial in the future generation of designer host proteins for improved resistance against ever-evolving pathogen virulence factors.

Highlights

Pectin, a galacturonic acid-rich complex polysaccharide found in all plant cell walls (CWs), is composed of homogalacturonan (HG), xylogalacturonan, apiogalacturonan, rhamnogalacturonan I, and rhamnogalacturonan II (Vorwerk et al, 2004)
Docking analysis showed that the concave surface of PglPGIP1 interacted strongly with the N-terminal region of AnPGII away from the active site, whereas it weakly interacted with the C-terminus of FmPGIII
JQ425039), which shared a nucleotide sequence identity of 84%, were isolated from pearl millet using primers based on the consensus sequence of known monocot pgips

Summary

Introduction

A galacturonic acid-rich complex polysaccharide found in all plant cell walls (CWs), is composed of homogalacturonan (HG), xylogalacturonan, apiogalacturonan, rhamnogalacturonan I, and rhamnogalacturonan II (Vorwerk et al, 2004). In the present study, in silico protein modelling, docking, and mutation analyses were carried out to explain the in vitro results, gain an understanding of the underlying structural basis of interaction, and predict the putative amino acids involved.

Results

Conclusion