Pathogen-Associated Molecular Pattern Active Sites of GH45 Endoglucanohydrolase from Rhizoctonia solani

A 207-amino-acid residue endoglucanohydrolase (EG1) belonging to the glycoside hydrolase 45 (GH45) from Rhizoctonia solani acts as a pathogen-associated molecular pattern (PAMP). However, the mechanism of EG1 inducing plant immunity is unclear. Here, we found that EG1 contains two domains related to its PAMP function. Transient expression showed that EG1-1, the mutation deleting 60 amino acid residues from the N-terminal, still reserved the PAMP function. Further truncation of EG1-1 obtained two truncating mutations: EG1-2, deleting seven amino acid residues from the N-terminal of EG1-1 (SPWAVND), and EG1-3, deleting five amino acid residues from the C-terminal of EG1-1 (GCSRK). Transient expression showed that the two truncating mutations EG1-2 and EG1-3 all lost the PAMP function. Site-directed mutagenesis of EG1-1 showed that the three amino acid residues (P, W, and D) in the region SPWAVND and the two amino acid residues (C and R) in the region GCSRK were involved in the PAMP function. The homology model showed that the two regions were located at a surface on the EG1 and structurally independent. These results demonstrate that there are two functional regions for the plant immune function of the EG1 released by R. solani, and the two functional regions are independent of each other.