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Review
. 2017 Apr;18(3):457-468.
doi: 10.1111/mpp.12412. Epub 2016 Jun 9.

The HopF family of Pseudomonas syringae type III secreted effectors

Timothy Lo  1 , Noushin Koulena  1 , Derek Seto  1 , David S Guttman  1   2 , Darrell Desveaux  1   2
Affiliations
Review

The HopF family of Pseudomonas syringae type III secreted effectors

Timothy Lo et al. Mol Plant Pathol. 2017 Apr.

Abstract

Pseudomonas syringae is a bacterial phytopathogen that utilizes the type III secretion system to inject effector proteins into plant host cells. Pseudomonas syringae can infect a wide range of plant hosts, including agronomically important crops such as tomatoes and beans. The ability of P. syringae to infect such numerous hosts is caused, in part, by the diversity of effectors employed by this phytopathogen. Over 60 different effector families exist in P. syringae; one such family is HopF, which contains over 100 distinct alleles. Despite this diversity, research has focused on only two members of this family: HopF1 from P. syringae pathovar phaseolicola 1449B and HopF2 from P. syringae pathovar tomato DC3000. In this study, we review the research on HopF family members, including their host targets and molecular mechanisms of immunity suppression, and their enzymatic function. We also provide a phylogenetic analysis of this expanding effector family which provides a basis for a proposed nomenclature to guide future research. The extensive genetic diversity that exists within the HopF family presents a great opportunity to study how functional diversification on an effector family contributes to host specialization.

Keywords: HopF; Pseudomonas syringae; type III secreted effectors.

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Figures

Figure 1
Figure 1
Ribbon diagrams of the SchFPph 1449B type III secreted effector (T3SE) chaperone (A) and HopF1Pph 1449B (B) with α‐helices depicted in red, β‐sheets in blue and loop regions in yellow. Spherical depictions of the ADP‐ribosyltransferase (ADP‐RT) catalytic residues, R72 in blue and D174 in yellow, are shown for HopF1Pph 1449B. (C) HopF homologue alignment. An amino acid alignment was performed using MUSCLE with representative Pseudomonas syringae HopF subgroup members. The chimeric subgroups, AvrRpm2 and HopBB1, were trimmed to the region of HopF homology retaining 104 amino acids from the N‐terminus. HopF homologue amino acid sequences were then aligned to the P. syringae HopF alignment. The Ralstonia and Acidovorax homologues were trimmed to regions of HopF homology, removing 89 and 34 amino acids from the N‐terminus, respectively. The glycine at position 2 is essential for myristoylation and is conserved in all sequences aligned, except HopF2c. This residue is not shown for the Ralstonia and Acidovorax sequences because of trimming of their N‐terminal sequences. The ADP‐RT catalytic residues, R72 and D174, are boxed in red (for sequences, see Supporting Information 3).
Figure 2
Figure 2
Phylogeny of the Pseudomonas syringae HopF type III secreted effector (T3SE) family. One hundred unique HopF alleles were obtained from a collection of 386 P. syringae strains and a codon alignment was performed using MUSCLE. For the chimeric subgroups, AvrRpm2 and HopBB1, the sequences were trimmed to the region of HopF homology retaining 312 bp from the 5′ end of the alignment. All sequences were then realigned using the trimmed sequences. The maximum‐likelihood tree was then built using the GTR model and gamma distributed with 1000 bootstrap replications. Although 268 HopF genes were found, identical alleles were not included in this tree for simplicity (see Table 1). The numbers in parentheses beside HopF specific members indicate the numbers of identical alleles including the member represented in the tree. Some plant hosts of HopF containing P. syringae pathovars are shown (see Supporting Information 1 and 2 for FASTA sequences).
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