Arabidopsis YELLOW STRIPE-LIKE7 (YSL7) and YSL8 Transporters Mediate Uptake of Pseudomonas Virulence Factor Syringolin A into Plant Cells

Abstract

Syringolin A (SylA), a virulence factor secreted by certain strains of the plant pathogen Pseudomonas syringae pv. syringae, is an irreversible proteasome inhibitor imported by plant cells by an unknown transport process. Here, we report that functional expression in yeast of all 17 members of the Arabidopsis oligopeptide transporter family revealed that OLIGOPEPTIDE TRANSPORTER1 (OPT1), OPT2, YELLOW STRIPE-LIKE3 (YSL3), YSL7, and YSL8 rendered yeast cells sensitive to growth inhibition by SylA to different degrees, strongly indicating that these proteins mediated SylA uptake into yeast cells. The greatest SylA sensitivity was conferred by YSL7 and YSL8 expression. An Arabidopsis ysl7 mutant exhibited strongly reduced SylA sensitivity in a root growth inhibition assay and in leaves of ysl7 and ysl8 mutants, SylA-mediated quenching of salicylic-acid-triggered PATHOGENESIS-RELATED GENE1 transcript accumulation was greatly reduced compared with the wild type. These results suggest that YSL7 and YSL8 are major SylA uptake transporters in Arabidopsis. Expression of a YSL homolog of bean, the host of the SylA-producing P. syringae pv. syringae B728a, in yeast also conferred strong SylA sensitivity. Thus, YSL transporters, which are thought to be involved in metal homeostasis, have been hijacked by bacterial pathogens for SylA uptake into host cells.

The role of bacterial phytotoxins in inhibiting the eukaryotic proteasome

“…Similar results were obtained in living plant cells by proteasome activity imaging and profiling experiments, in which SylA seemed to target preferentially proteasomes in the nucleus [45]. As recently elucidated in A. thaliana, SylA is efficiently taken up into plant cells by YELLOW STRIPE LIKE7 (YSL7) and YSL8 transporters [46], which belong to a plantspecific oligopeptide transporter subfamily thought to be involved in transport and distribution of metal-nicotianamine complexes [47]. SylA is the founding member of a new structural class of proteasome inhibitors dubbed syrbactins, which is characterized by the 12-membered macrolactam ring and its functional α,β-unsaturated carbonyl group [48,49].…”

“…This difference determines whether and how syrbactins are taken up into cells. GlbA has antifungal activity [62] and evidently must be taken up by fungal cells such as yeast, whereas yeast is completely insensitive to SylA [46]. However, yeast can be rendered sensitive towards SylA by expression of the A. thaliana YLS7 and YSL8 plantspecific transporters [46].…”

Proteomic Analysis of the Soybean Symbiosome Identifies New Symbiotic Proteins*

“…YSL proteins in dicots typically transport metals such as iron, copper, and manganese complexed with nicotianamine (NA) (reviewed in 129,130). However, the closest Arabidopsis homolog, YSL7, has recently been shown to transport the Pseudomonas virulence factor, Syringolin A, which is a peptide derivative, with transport of Syringolin A inhibited by tri-to octapeptides (131). Syringolin A has similar chemical properties, size and net charge to metal-NA complexes (131) that are the usual substrate for YSL transporters, but whether AtYSL7 can also transport metal-NA was not established.…”

Soybean Yellow Stripe-like7 is a symbiosome membrane peptide transporter essential for nitrogen fixation

“…In particular, members of one phylogenetic clade of the YSL family, including YSL5, 7 and 8 (Group III) are not well characterized. A recent study showed that Arabidopsis YSL7 and 8 are responsible for the import of a Pseudomonas syringae virulence factor, syringolin A (Syl A), into the plant cytoplasm where it inhibits the proteasome (Hofstetter et al 2013). Syl A is a peptide derivative and peptides of 4-8 amino acids in length were able to inhibit its transport in plants and in yeast expressing AtYSL7.…”

“…Syl A is a peptide derivative and peptides of 4-8 amino acids in length were able to inhibit its transport in plants and in yeast expressing AtYSL7. Consequently, it was suggested that AtYSL7 and AtYSL8 act as oligopeptide transporters, although direct evidence of oligopeptide transport was not shown (Hofstetter et al 2013).…”