term Hop applies generically to proteins that are secreted and/or translocated
by the TTSS of P. syringae and related plant pathogens.Most are EFFECTORS,
with their primary function being inside the host cell. Others are likely to be
TRANSLOCATORS or HELPERS,
assisting the delivery of true effectors across host cell barriers. This
nomenclature system does not distinguish between effectors and translocators.
All proteins traveling the TTSS pathway will be classified as Hops.
The following criteria have been designed to ease the experimental burden required
for Hop naming, while minimizing the generation of artifactual Hops and Hop families
that are not expressed or secreted. In order for newly identified proteins to
receive a Hop designation, ONE OF THE FOLLOWING CRITERIA
MUST BE MET:
Phylogenetic Membership in an established Hop family in combination with a consensus
>60% of a new protein's sequence can be aligned (e<10-5)
with one or more members of a Hop family previously characterized using criteria
B, C, or D, the new protein can be given a name reflecting this relationship.
However, more in-depth phylogenetic analyses are encouraged for a more accurate
assessment of family and subgroup membership (see protocol
for phylogenetic analysis). The additional requirement of an N-terminal secretion/translocation
sequence (see criterion B below) is included so that Hop names will be limited
to plausible TTSS substrates. If a protein or gene is found to be homologous to
a previously identified Hop family, but upon testing, does not meet any of the
three experimental criteria, the name should be modified to conform to the guidelines
for pseudogenes and non-secreted/translocated homologs (see Rules
for Name Selection).
Confirmed HrpL-dependence and a consensus translocation motif
Hop identification has typically
proceeded with a rapid discovery phase followed by a slow phase of experimental
confirmation, leading to a situation where proteins are published as "TTSS
effector candidates", with experimental confirmation and final naming delayed
for a year or more. To allow naming to proceed at a faster rate and eliminate
the need to name candidate Hops, proteins will be considered eligible for the
Hop designation if HrpL-dependence is experimentally demonstrated and a consensus
N-terminal targeting pattern is present. Based on the findings of previous investigations,
the consensus N-terminal targeting pattern is defined here as having:
- > 10% Ser
in the first 50 amino acids
Ile, Leu, Val or Pro in the third or fourth position
Asp or Glu residues in the first 12 amino acids
It is expected that effectors named using criterion C will eventually be tested
for Type III-dependent secretion and/or translocation.
more information on the identification of a consensus translocation motif, see
Guttman et al, 2002, Petnicki-Ocwieja
et al, 2002, Greenberg and Vinatzer, 2003
Hrp-dependent (TTSS) Secretion and/or Translocation and Evidence of Expression
Passage through the TTSS
is the defining characteristic of Hop proteins; however, evidence of expression
is also required to avoid the proliferation of Hop families that have been computationally
identified and translocated under artificial conditions, but for which there may
be no actual expression.
of expression includes bioinformatic or experimental evidence supporting expression
of at least one member of the Hop family in question. Evidence of expression should
also include assessment of the presence of an upstream Hrp box.
Innes et al. 1993 (J. Bacteriol. 175:4089), Shen and Keen, 1993 ( J. Bacteriol.
175:5916), Xiao and Hutcheson, 1994 (J. Bacteriol. 176:3089), Fouts
et al. 2002)
methods for demonstrating translocation include showing that AvrRpt2 or Cya can
be translocated into the cell interior in a Hrp-dependent manner, when fused to
either the N-terminal portion of the candidate protein or to the full ORF of the
candidate being tested
(see Mudgett, et al,
2000, Guttman, et al, 2001, Schechter,
et al 2004).
methods for evaluating secretion include use of Western blots to detect epitope
tagged proteins in the culture supernatant
et al 2002).
shown to induce an avirulence reaction on plant hosts when expressed in heterologous
strains of P. syringae will be considered Hops. However, subsequent demonstration
of secretion or translocation is encouraged, given that proteins such as AvrD,
which direct the production of low molecular-weight elicitors, are active in bacterial
cells but may not be TTSS substrates.