Identification and characterization of glucanase inhibitor proteins: Uncovering the molecular arms race in the plant cell wall

The plant cell wall represents the interface with microbial pathogens and so it is not surprising that many wall-localized mechanisms for surveillance, attack and defense have been identified. We have recently discovered a new class of proteins, termed glucanase inhibitor proteins (GIPs) that are secreted by species of Phytophthora into the plant wall during pathogenesis. GIPs bind and inhibit the activity of plant extracellular endo-beta-1,3-glucanases (EGases), thus blocking the release of glucan elicitors. Several GIPs were first identified in P. sojae and they appear to show a high specificity for particular EGase isoforms since GIP1 binds specifically to the EGaseA isoform from soybean, but not to another isoform, EGaseB. Despite their potential importance as suppressors of EGase-mediated defense responses, the molecular basis of GIP action and specificity are not well understood. We have recently identified a 4-member GIP family from P. infestans, and detected GIP isozymes both in culture and in vivo into the apoplast of P. infestans-infected tomato leaves using Western analysis. Molecular modeling has been used to predict putative docking sites on the surfaces of EGases and GIPs that may be involved in the high affinity binding between these proteins and the results suggest that positive selection has driven the co-evolution of these protein families. This further suggests the existence of a molecular arms race between GIPs and EGases.