The TAL Effector AvrBs3 from Xanthomonas campestris pv. vesicatoria Contains Multiple Export Signals and Can Enter Plant Cells in the Absence of the Type III Secretion Translocon

Abstract

Pathogenicity of the Gram-negative plant-pathogenic bacterium Xanthomonas campestris pv. vesicatoria depends on a type III secretion (T3S) system which translocates effector proteins into plant cells. Effector protein delivery is controlled by the T3S chaperone HpaB, which presumably escorts effector proteins to the secretion apparatus. One intensively studied effector is the transcription activator-like (TAL) effector AvrBs3, which binds to promoter sequences of plant target genes and activates plant gene expression. It was previously reported that type III-dependent delivery of AvrBs3 depends on the N-terminal protein region. The signals that control T3S and translocation of AvrBs3, however, have not yet been characterized. In the present study, we show that T3S and translocation of AvrBs3 depend on the N-terminal 10 and 50 amino acids, respectively. Furthermore, we provide experimental evidence that additional signals in the N-terminal 30 amino acids and the region between amino acids 64 and 152 promote translocation of AvrBs3 in the absence of HpaB. Unexpectedly, in vivo translocation assays revealed that AvrBs3 is delivered into plant cells even in the absence of HrpF, which is the predicted channel-forming component of the T3S translocon in the plant plasma membrane. The presence of HpaB- and HrpF-independent transport routes suggests that the delivery of AvrBs3 is initiated during early stages of the infection process, presumably before the activation of HpaB or the insertion of the translocon into the plant plasma membrane.

Show me your secret(ed) weapons: a multifaceted approach reveals a wide arsenal of type III‐secreted effectors in the cucurbit pathogenic bacterium Acidovorax citrulli and novel effectors in the Acidovorax genus

“…generation of reactive oxygen species due to injury caused by the infiltration treatment (Huang et al, 2019;León et al, 2001), it is possible that this effector possesses additional, T3S-independent export signals that allow its entering into the plant cell, as recently shown for the X. euvesicatoria effector AvrBs3 (Scheibner et al, 2017).…”

A conserved motif promotes HpaB‐regulated export of type III effectors from Xanthomonas

“…Notably, non‐effectors can be translocated by Xcv strains lacking the export control proteins HpaB and HpaC, respectively (Büttner et al ., ; Scheibner et al ., ). Studies in Xcv and Erwinia amylovora , using serial N‐terminal lengths of T3Es fused to translocation reporter proteins, located the putative translocation signal in N‐terminal protein regions downstream of the T3S signal (Mudgett et al ., ; Oh et al ., ; Scheibner et al ., ). Based on sequence comparison of nine Xcv T3Es, a consensus translocation motif (TrM) has been proposed (Escolar et al ., ), which served as a starting point for the present study.…”

Split-TALE: A TALE-Based Two-Component System for Synthetic Biology Applications in Planta

“…We hypothesized that a weaker expression rate of the BC might also reduce its background activity. Furthermore, in our first design the BC contains the complete NTR, which was reported to contain the translocation signals (1-50 amino acids) and an essential part of the DBD in terms of transcriptional induction (64-288 amino acids; Szurek et al, 2002;Schreiber et al, 2015;Scheibner et al, 2017).…”

“…1A). The NTR contains signals that promote secretion and translocation, as well as essential parts of the DNA-binding domain (Gao et al, 2012;Schreiber et al, 2015;Scheibner et al, 2017). Structural analysis revealed that the repeat domain is extended into the NTR by at least 4 degenerated repeats (called Repeat-3 to 0; Deng et al, 2012;Gao et al, 2012;Mak et al, 2012).…”