γ-Conglutin, the Lupinus albus XEGIP-like protein, whose expression is elicited by chitosan, lacks of the typical inhibitory activity against GH12 endo-glucanases

Abstract

gamma-Conglutin, a glycoprotein from Lupinus albus seed, has been characterized at molecular level but its physiological function is still unknown. gamma-Conglutin shares a high structural similarity with xyloglucan-specific endo-beta-1,4-glucanase inhibitor proteins (XEGIPs) and Triticum aestivum xylanase inhibitor (TAXI-I), which act specifically against fungal glycosyl hydrolase belonging to families 12 and 11, respectively. To assess the possible involvement of gamma-conglutin in plant defense, germinating lupin seeds were incubated with chitosan. The relative quantification of gamma-conglutin mRNA extracted from cotyledons was then carried out by RT-qPCR and indicated that chitosan strongly elicited the expression of gamma-conglutin. Moreover, biochemical trials aimed to test the inhibitory capacity of the protein have been also carried out. gamma-Conglutin failed to inhibit representative fungal endo-glucanases and other cell wall-degrading enzymes. To explain the lack of inhibitory capacity we investigated the possible structural differences between gamma-conglutin and XEGIPs and TAXI-I, including the construction of a predictive 3D model of the protein. Bioinformatic analysis suggests that the lack of inhibitory activity of gamma-conglutin can be attributed to sequence differences in the inhibitor interaction domains, and in particular to a sequence deletion in one of the functional loops.