Agrobacterium tumefaciens C58C1 strains carrying the vector pBin-Hyg-Tx, pBin::nopT1, and pBIN::nopT2 were infiltrated into N. tabacum cv. Xanthi and N. benthamiana leaves. NopT1 elicited localized cell death in both Nicotiana species (Fig. 4b). By contrast, leaves infiltrated with A. tumefaciens carrying pBin::nopT2 did
not show any visible symptoms (Fig. 4c). No visible symptoms of cell death were observed when Agrobacterium with an empty vector was infiltrated (Fig. 4a). In light of these results, further studies focused on the analysis of NopT1 function. To determine whether the putative catalytic triad (C/H/D) of NopT1 is required for the HR-like cell death in tobacco, we constructed substitutions at positions 100 (C100S), 213 (H213A), and 228 (D228A) with Ala (Fig. 2d). Ibrutinib Dasatinib clinical trial The coding regions of the site-directed mutants were subcloned into a binary Agrobacterium vector and tested for ability to elicit the HR in N. tabacum and N. benthamiana when overexpressed directly within the plant cells via the Agrobacterium-transient expression system. None of the mutants elicited cell death (Fig. 4e–g), whereas the wild-type NopT1 elicited a strong HR (Fig. 4b). We also
examined whether the site-directed mutants retained enzymatic activity. As shown in Fig 2b, all site-directed mutants had lost the NopT1 processing in E. coli, although not completely and their in vitro enzymatic activity ID-8 was significantly reduced in comparison with wild-type protein (Fig. 3c). These results corroborate further the prediction that that NopT1 is a cysteine protease and requires an intact catalytic triad for both enzymatic and HR-eliciting activity. Previous studies have shown
that all YopT/AvrPphB family members identified so far contain an embedded consensus site for eukaryotic fatty acylation which may be exposed following autoproteolytic processing of these effectors (Puri et al., 1997; Nimchuk et al., 2000; Dowen et al., 2009). Similarly, NopT1 possesses putative sites (Fig. 1b) for both N-myristoylation (G50) and S-palmitoylation (C52 and C53) that lack experimental validation. To investigate whether these acylations play a role in cell death elicitation by NopT1, we made deletion and site-directed mutants affecting either one or both sites. Initially, we made a deletion mutant, Δ50N, in which an ATG codon was introduced just before the A51 codon by replacing the glycine (G) residue at position 50 by a methionine (M) residue. Transient expression via agroinfiltration of this mutant displayed identical necrotic phenotype to that elicited by the full-length protein, in terms of both timing and intensity of the necrotic response (Fig. 4d). Although myristoylation of NopT1 has not been demonstrated biochemically, it is tempting to speculate that an intact myristoylation motif may not be required for HR elicitation by NopT1 at least in plants tested.